NO visa-on-arrival in Gabon (airport)
 

Contrary to some overland travellers who recently received visas-on-arrival at Gabon land borders, the airport in Libreville still imprisons, blackmails and deports Western travellers:
Gabon blackmails and imprisons high-end tourist - Gentleman Adventurer

President Bongo's talk about opening up for high-end tourists is all bullshit. Or he lost control over his staff.

with the land bourder you have some flexability, if you treat the cameroon side well, then they can issue a transit visa to re enter cameroon if gabon does not play ball...then you can take the logging roads to congo