These are species with at least one recorded territory in Gabon, not a national checklist: a bird list from eBird or Avibase is more complete. The counts follow recording effort as much as wildlife, so a country with more visitors and more naturalists will show more of both.
The species with the most open occurrence records in Gabon, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Gypohierax angolensis

Psittacus erithacus

Turdus pelios

Lophoceros fasciatus

Merops malimbicus

Hybomys univittatus

Bycanistes fistulator

Anthus pallidiventris

Anabathmis reichenbachii

Merops variegatus

Ceratogymna atrata

Halcyon senegalensis
The reference database has no regions filed under Gabon.
The field guide covers Gabon on the map, but none of its species have an entry written for them yet.