I am afraid there are no outfitter at all:

I found
a Belgian :
Henri MARTIN
6A, rue du Pont - B 4180 HAMOIR BELGIQUE
TI 00.241.59.19.05
Mobile: 00.32.477.66.52.59

and a Pole
contactpologne

but they don't look very serious.
In fact Gabon was an excellent hunting country in the past. Our past french president sailed (too) often there hunting Elephant, buff, sitatunga and such.

The trouble is that the (rain) forests (jungle, very wet) are now over-worked. That means that there are too many roads facilitating the penetration of the poachers.
In Gabon poaching is a leading industry. In these conditions hunting is almost impossible.
I was told that the Gabonese President, Omar Bongo, entertain his hush guests with hunting the National Parks with his own helicopter.
Sadly, if You aren't a prominent fat cat You have to consider NOT hunting in Gabon.
Perhaps am I inaccurate.
The best guy around the block is WINK on this forum. He intimately knows Africa more than his pocket. I think his opinion will not differ from mine.
Good luck