I also purchased a pair of GBRs from APB about a month ago. They said they couldn't guarantee a sexed pair...and I'm also still having trouble sexing them based on coloration.
But they do constantly follow each other around and dig nests in the substrate, so I'm assuming I have a sexed pair. Not sure if this is a fair assumption?
On a side note, APB sent really nice looking fish, except one was missing a pelvic fin (a la Nemo)...which is a bummer.