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.