[3.P] Gestalt Advice

[Macabros]

Ok so I wanted to make a Gestalt Build and I need advice

One side is going to be
Spirit Lion/Whirling Frenzy Barbarian 1/ DC Fighter 4 (or 6)/Warforged Juggernaut 5/DC Fighter 2 (If I took 4 at the first side) (total of 12)

I am not sure what to put on the other side. But I considered the Iron Priest Archetype of Cleric.

Obviously Warforged Race

[Jack_Simth]

Ok so I wanted to make a Gestalt Build and I need advice

One side is going to be
Spirit Lion/Whirling Frenzy Barbarian 1/ DC Fighter 4 (or 6)/Warforged Juggernaut 5/DC Fighter 2 (If I took 4 at the first side) (total of 12)

I am not sure what to put on the other side. But I considered the Iron Priest Archetype of Cleric.

Obviously Warforged Race
Well, here's some generic advice:

You've still only got the one set of stat rolls and equipment.
You want to be as SAD (single attribute dependant) as is feasible. Sure, the Sorcerer//Monk might look good on paper... but if you try to be both a Monk and a Sorcerer, you need: Strength for Damage, Dex for AC and ranged attacks, Con for HP, Wis for AC and class features, Charisma for spells. Int for skills, too. That's VERY MAD - you won't be able to keep decent scores in all of them, and you won't be able to afford stat-boosting items for all of them (if the character picks one, however, it'll be OK - especially with feats like Ascetic Mage to cut down on the MAD, and spells like Polymorph to fix the problems with the physical stats). Something like a Druid//Ninja, on the other hand, gets it's Strength and Dex from Wildshape, has a base 6 skill points per level, doesn't really need Charisma, and so can do just fine on nothing more than Constitution and Wisdom.

Be aware of power curves.
At low levels, Melee-Based characters have a rather lot of narrative control. At higher levels, they peter out.
At high levels, Full-Casting characters have a rather lot of narrative control. At lower levels, not so much.
Skill-Based characters, on the other hand, tend to be about mid-range for the entire span of levels.
Generally speaking, you want at least one "side" of your character to have at least a mid-range power curve. Melee + Full Caster is very common, and works mostly well, as does Skill + Full Caster. Skill + Melee will do great at lower levels and OK at higher levels; Skill + Full Caster will do OK at lower levels and great at higher levels.
As always, Druid works great at all levels, being Full Caster, part skillmonkey, and and a decent meatshield already.

You've still only got the one set of actions.
You've only got one set of actions: Standard, Move, Swift, and some unspecified number of free (Unless you've got something that says otherwise, such as a Factotum-8, of course). Generally speaking, you want one 'side' to be active (use up your standard and move actions, sometimes swift as well), and the other 'side' to be passive (non-action defences and boosts to your offence). The Fighter//Warmage, for example, has a problem: He can blast, or he can hit things... and his ability to blast does not much help his ability to hit things, and vice versa. Take the Druid//Ninja, on the other hand: Casts some long-duration buff spells and Wildshapes into something with Pounce pre-combat (not necessarily in that order); in combat, swift-action invisibility and Charge for Sudden Strike on 5+ attacks. The Druid side is mostly active (consuming actions with the charge), the Ninja side is mostly passive (Sudden Strike + Swift-action invisibility). However, the Druid side is slightly passive (long-duration buffs, Wildshape). In this example, the Full Casting aspect is both active and passive - long duration buffs in Wildshape and some spells, using the actions to pounce in battle. A Full Caster can be a passive side - you get just enough of your casting stat to cast all of your buffs, and then use long-duration ones that you can cast outside of combat.

Be warned: Some class features interfere with each other.
Arcane spells don't work so hot in full plate. If you're going to try for a Fighter//Wizard, you've got the problem that you can't really cast spells in your heavy armour. The Fighter//Druid can't really use the better armours (until Dragonhide becomes affordable, anyway), and has a related problem. There's a fair number of classes that don't mix well because their abilities clash. Watch out for that (mostly comes up with armour and shields, but there's a handful of others).

Know how your DM handles Monstrous Races.
That might be "Play a standard race" (No monstrous races).
That might be "The racial adjustment costs both sides" (almost the same thing, but worth it for a very small handful of creatures)
That might be "The racial adjustment comes first and costs both sides, but is half and half" (might be worth it, with some critters).
That might be "The racial adjustment takes up one track" (under this circumstance, you're pretty much crazy NOT to take something with a strong LA [CR, in Pathfinder]).

In your case? Yeah, Cleric is a pretty fair idea. Given that you started with a melee-centric build, you'll probably want to focus on long-duration and swift-action buffs. Quicken Spell will be a good feat for you, I imagine, as would the 3.5 version of Persistent Spell (later). Divine Metamagic (3.5 Persistent Spell) is going to be useful for you, but less so than for other races due to the Charisma penalty for Warforged.

[Macabros]

I figured it would give a good balance as the fighter feats could get you any combat feats you need and using the PF progression of feats to pick up any feat for Cleric I might need.

But is there any feats beyond the two listed above that i would need to focus on?

[Jack_Simth]

I figured it would give a good balance as the fighter feats could get you any combat feats you need and using the PF progression of feats to pick up any feat for Cleric I might need.

But is there any feats beyond the two listed above that i would need to focus on?
Requirements for same. Crafting feats are good (especially with a Pathfinder base, where you don't have to worry about XP components and can substitute Spellcraft checks for required spells for most items), but... well, Cleric is a full caster, and can fully swap out spells on a daily basis. If you remember to arrange things so that you'll have at least a 19 in Wis by 17th level... it's pretty hard to mess their builds up too much. Quicken Spell and 3.5 Persistent Spell are really just there so that you don't need to spend combat rounds buffing when you could instead be doing your Pouncing Full Attacks - which appear to have been the impetus for the build. Sure, there's things I could go over to make the build have MOAR POWAH! ... but that's not necessarily fun. Worse, if you're too terribly out of line with the rest of the players at the table, you'll also ruin their fun. More character power optimization is not necessarily a good thing.