If you have a 2011 Trek 7.5 fx you will not be able to install these grips. Trek used a custom Bontrager Satellite Plus IsoZone handlebar and the bar is not solid the whole way thru. I assume you went with the GC3 because you wanted to add bar ends. To do this you will need to replace the whole handlebar. I have a writeup about replacing the handlebars on my blog
http://bartsbiking.com/?p=510
It has been confirmed that Trek will be coming out with bar ends compatible with these handlebars this summer, but i have not heard a date yet.
If you have a 2010 or lower you wont have any issues and ignore this post.