Gemfire and Atomikos did not play well together when we tried this. It was 6 months ago though and things may have changed.
http://communities.vmware.com/thread/437150
Honestly we gave up on using it for use cases such as this.
The idea that you can have transactions, but not if you want to persist the data to disk was a little unsettling.
"You run a lot of regions so you get data integrity that way" sounded way too much like "hope your data centers don't suffer a power outage" to me.
Hope is no way to run a business.