Gabbe, stop trolling. You have no obligations to pay any royalties or share any profits with the Glest team, as long as you follow the license on Glest (the GPL). Mind you, most commercial games aren't released under the GPL, and I would personally advise / prefer you work on something FOSS instead of a commercial adaption heavily based on a FOSS program...
Of course, you can also make an optional donation to the Glest team, if you wish.