If you're going to Boma for dinner, you could always spend the day at AK and then take the short bus from AK to AKL. To get back to GF, hop on the first bus going to a park and then transfer there to GF. Of course, if you're having breakfast, that's another scenario.
Be advised: it took us well over an hour to get back to the Poly after dinner at Boma one evening in July. You could also take a taxi. We saw several just sitting out on front of AKL. In retrospect, we should have done that.