gnome cunning: advantage against magic. What count as magic?

[Myth27]

Is it only spells? what about monsters abilities?

[Catullus64]

It works against all magical abilities, monster included. Compare that to, say, a Mantle of Spell Resistance, which singles out saving throws against spells.

That said, 5e is a bit wooly on the subject of Magic vs. Not-Magic, in contrast to, say, 3e/3.5, with its tags of Magical/Supernatural/Extraordinary, which made lots of clean delineations. Lots of monsters have ablities which seem to be a result of the creature's explicitly magical nature, like a Mephit's Death Burst, a Myconid's Spores, or a Dragon's Breath Weapon, to say nothing of creatures with psionics, but it's not clear whether a feature that grants "advantage on saving throws against magic" would apply to these. (I have definite opinions on all of these cases, but I cite them as examples where there could be ambiguity.)

[Millstone85]

The Sage Advice Compendium gives the following method:

Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Is it fueled by the use of spell slots?
• Does its description say it’s magical?

If your answer to any of those questions is yes, the feature is magical.