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It's a matter of opinion, honestly.
3.5 illusion spells are kinda unbalanced compared to other schools. There's some really good ones, with some really bad ones, and not much in between.
Or, that's what some people think.
Others see illusion as a very versatile school that takes skill as a player to make optimum use of, but when that's the case it can be the most potent school.
With that in mind, a gnome illusionist in early game is going to have a nice edge from the racial fe at. If they stack more spell focus, even if it's just 1 more, a +2 to DC can be a major benefit even in the high teen levels.
I've had a player run a gnome illusionist, and it can be a fun class/race combo. A lot of layered fakery, distractions, controls. You use the class right, and you can control a battlefield with almost no risk. Since the will save on most illusion spells is opposed by wizards and clerics rather well, a point or two to the DC can be enough to dominate combat. If you can befuddle the ones most likely to see right through your efforts, the rest have way less chance.
But, you'd only benefit if you went that route. Illusion as a school has other aspects, and the trickery side of things might not be what you want, so taking Gnome only because if the free spell focus wouldn't do much