George Gershwin's virtuosic treatment of his own song is a landmark of American 20th-century piano literature. It is distinctive for its fusion of classical variation form with syncopated, idiomatic jazz rhythms.
Mastering the cross-rhythms and the specific syncopated articulation of the jazz style is the central difficulty.
Comparable difficulty and character — good candidates for what to learn next.
'I Got Rhythm' Variations is rated Henle level 8 of 9 — concert repertoire, roughly diploma level. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 8 — meaning you can already play level 7–8 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 'I Got Rhythm' Variations lasts about 8 minutes.
'I Got Rhythm' Variations is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Rhapsody in Blue (Solo Piano) by George Gershwin, Hungarian Rhapsody No. 2 in C-sharp minor by Franz Liszt, Rondo a capriccio in G major by Ludwig van Beethoven — see the full similar-pieces list on this page.