Op 81a is also a very heavy hitter. Personally, I'd advise it over Op. 111, which is more popular among students from what I've gathered.
Gaspard de la Nuit is definitely NOT in the Romantic period; Ravel was an Impressionist (or Post-Impressionist if you're a strict and decisive music historian), so everything he wrote is more than likely gonna be put into that category.
You could ask about programming the entire Rach Op. 16. Or a Liszt Hungarian Rhapsody or something. Schumann also has some options, as does Chopin (the ballades and scherzi are common picks, but you could do something like the Op. 49 Fantasie or the Op. 61 Polonaise-Fantasy).
Etudes are a wide category. Typically grad schools require 1 Chopin and 1 other challenging etude by a different composer, don't they? If that's the case, a Chopin etude and a Ligeti etude would be great contrasts. Also consider Rachmaninoff etudes tableaux- but if you program the Op. 16, I'd stay away from them (too much of one composer, could throw off your audition). If you're going to (have to) do a Chopin etude, I'd pick a hard one than the black key etude (or any of the G flat etudes, there's one in Op. 25). I'd go with one of the big ones; Op. 10/1, 2, 4, 7, or 10, and Op. 25/3, 4, 6, 8, 10, or 11 (12 is a bit less daunting and music schools know this; but if you can pull it off, by all means!).
For 20th century, Ligeti would be a good choice. Also look into Medtner if you want something more romantic.
Good luck on your audition!