Then, you need to be lenient about naming-collisions. You don't want to disqualify a really cool feat entry just because there is some warlock power with the same name (or some other technicality that makes it GSL incompatible). Just make sure the entry doesn't violate the spirit of the GSL ("This feat is for use with my house rule where hit points are replaced with a damage threshold...").
Good point. Skip the GSL thing but it will have to be something that will work with the core rules or any other material specifically listed. Like if Goodman has something coming out that could work with that month's theme. Anything that will work on its own as is, no house rules.