It doesn't really answer your question, but most people cross into Gabon from Cameroon and then onto the Congos. There are a few reasons, but primarily Gabon is quite a nice country and definitely worth visiting.
As for the roads directly into the Congo, I was under the impression from 4x4'ers I met in Brazza that the road(s) were very tough slogging for vehicles and thus most likely tough going on bikes. Weather, wet or dry season, also has a big impact on many routes in general.
Once you get into Gabon, you have the choice to depart on the eastern route through Franceville or the southern route. Franceville is an interesting route, a bit sandy at times but there is an interesting national park to cross through. Happy riding.