Geni is another shared world tree, so what you export is the part around you rather than a private tree.
Where it is
Geni's GEDCOM export is reached from a profile's Actions menu: choose Export GEDCOM. It does not require a Pro subscription. Geni builds the file and emails you when it is ready.
What is in the file
People, relationships, dates and places within the limits Geni allows. The limits are based on contribution rather than generations: the export covers profiles you added (plus immediate family), capped in proportion to how many profiles you have contributed, with a higher cap for Pro accounts. Some files arrive with a single generic source ("Geni World Family Tree") attached to many facts, which is Geni's way of saying where the data came from, not a real citation.
What is not
Photos, documents, and DNA data. Living people you do not manage are limited.
Next
Open the file in GED-VIEW and look at the Issues page. Shared trees pick up duplicates and impossible dates over time, and this is a quick way to see what came along.