Gauths are the beholders' lesser (and much freakier looking) cousins, armed with a plethora of special abilities. They also seem incredibly fun to play to me. Let's take a look at them, shall we?
Despite the great similarities, gauths are not true beholders and as such can't benefit from most of the support LoM gave beholders (this includes Beholder Mage). Still, certain feats *cough*Metaray*cough* remain available.
To put it simply: a gauth's natural abilities are strong. Your eye rays alone allow you to deal direct damage (Inflict Moderate Wounds, Scorching Ray), strip away magical effects (Dispel Magic), debuff foes (Ray of Exhaustion) or even force save-or-loses (Sleep, Paralysis). And that's before taking into account your high ability scores, stunning gaze, nonmagical flight, bonus feat, all-around vision, and natural armor. Your versatility is surprisingly large for a non-spellcasting monster.
Also good to remember is that your two primary offensive abilities (eye rays and stunning gaze) can both be used as free actions. This means you can spend your entire turn moving, re-targetting someone with your gaze, or using whatever class features you pick up later on while still being able to employ your natural abilities. That's awesome.
So now that we've established that gauths are pretty good an sich, how do they compare to other classes when class levels get added? Surprisingly good for a race with no advanceable abilities.
You see, something like a hag gains several useful spell-like abilities as part of her race. At higher levels, however, these abilities will have lost most of their uses, or will have become available as magic items. This makes most level-adjusted races far less useful at higher levels.
The gauth, however, can use his racial abilities together with whatever he learned from his class levels, because these racial abilities don't use up actions. A gauth wizard can unleash a handful of save-or-sucks a turn. A gauth rogue can fire sneak attack-enhanced rays while hurling a +1 returning mouthpick dagger. A gauth cleric can heal his undead hordes while buffing himself and blasting his foes. Where another race would only give more options, a gauth adds those options to your conventional turn.
So what LA is appropriate here? Call me crazy, but I'm going for +4. A higher LA may even be warranted, but I didn't want to turn gauths into complete glass cannons.
Casting two save-or-loses a round, at will, is a powerful ability that shouldn't be underestimated. A 10th-level wizard may be able to pull off something similar a few times per day, by combining something like a Quickened Seething Eyebane and Slay Living. Both this combo and the gauth's rays are effective against most creatures, and the save DC's will be about equal. The wizard has more versatility, the gauth can use the combo at-will and can use other actions in conjuction with the rays, which seems about even.
Finally, allow me to suggest taking dragonfire adept levels. Your breath weapon will be an excellent way to make use of your actions, you have constitution and charisma synergy, and most of all you'll be a firebreathing sphere of doom.