Gabon sits on the equator, so it is hot and humid year-round. Temperatures hover between 24C and 32C regardless of the season. What changes is the rain.
The long dry season runs from June to September and is the best time to visit. Rainfall drops significantly, forest trails are more passable, and wildlife gathers around remaining water sources. July and August are the driest months. This is also whale-watching season, with humpbacks visible off the coast at Loango and Mayumba.
The short dry season in December and January brings a brief respite from the rains. Forest fruit is abundant during this period, drawing gorillas and elephants closer to the coast, so wildlife viewing can be excellent. Leatherback turtles nest on the beaches from November through mid-January.
The long rainy season from February to May dumps the heaviest rainfall. Many forest roads become impassable, and some parks are difficult or impossible to reach. Libreville itself gets very wet, with over 300mm of rain in March alone.
The short rainy season in October and November is less intense but still wet enough to make inland travel challenging.
If you have flexibility, aim for July or August. If you want to see nesting turtles, come in November or December and accept that you will get rained on.