There is a thread in another forum, but it would be inappropriate to post the link on this forum.
Just google 'GMAT, Geneva', and it should be listed in the results.
How useful the posts are is another matter ...... and it's a bit second hand.
For more general results, try 'GMAT, Switzerland'.
But here's a previous thread from this forum: Gmat test - personal experiences???
Ain't google, great? (Even for academics).