Map from Google - Gabon

In Gabon, an African country crossed by the equator, the climate is hot and humid all year round in the north and in the inland areas, with a short dry season from June to August, while in the south-central coast, the temperatures become cooler during the dry season. In the far south the dry season is even longer, going from mid-May to mid-September.
The temperatures vary little over the course of the year in the north and center of the country: the warmest period is from January to May, when highs are around 86 °F, while from June to September it drops a bit, hovering around 75/77 °F on the south-central part of the coast, and to 81/82 °F in the inland areas.
The annual precipitation in most of the country ranges from 60 to 80 inches, while it's close to 120 inches in the northern part of the coast where the capital Libréville is located. In the northern inland areas, near the equator (see Bitam, Oyem, Makakou), there are two rainy seasons, from March to May and from September to November, related to the two passages of the sun at its zenith.
The majority of Gabon is occupied by forests and mangroves, except the extreme east, which is occupied by the savannah. There are several national parks, designed to protect the biodiversity of the region.
The amount of sunshine is never great, and it's even worse in the dry season, from June to August, when the sky is often cloudy. The relatively sunnier period runs from December to February.
In the interior there are hilly areas, where the heat is tempered by altitude. Here are the average temperatures of Bitam, situated in the far north, at 2,000 feet above sea level.
Average temperatures - Bitam
Min (°F)686868686868666668686868
Max (°F)848686868682818182848484

As mentioned, in the north there are two rainy seasons, and as a result there will be two relatively dry periods, from December to February and from June to August. Here is the average rainfall in Bitam.
Average precipitation - Bitam
Prec. (in)

The capital Libréville is located on the northern part of the coast. Here are the average temperatures.
Average temperatures - Libréville
Min (°F)757373737573727273737375
Max (°F)868888888684828284848686

In Libréville, 102 in of rain per year fall, with a single dry season from June to August, when it almost never rains. Here is the average rainfall.
Average precipitation - Libréville
Prec. (in)8.111.410.415.69.61.600.

In the south, the winter cooling is more noticeable, so much so that the maximum temperatures drop to 79/81 °F in July and August. Here are the average temperatures of Tchibanga.
Average temperatures - Tchibanga
Min (°F)727272727268646670727272
Max (°F)868888908682817982848684

In the south the dry season is longer, so that it begins in mid-May and lasts until September. Here is the rainfall in Tchibanga.
Average precipitation - Tchibanga
Prec. (in)

White Canyon, Gabon

For swimming, the sea is warm all year round, even though the water temperature drops a little from July to September, when it goes down to around 77 °F at Libréville, and around 73 °F in the south-central part of the coast.
Sea temperature - Libréville
Sea (°F)828484848279777979818182

When to go

The best time to visit Gabon is from June to mid-September, and in particular the months of July and August, because they are the least rainy, and also the ones in which in the past there has been no anomalies in the rains; during this time the heat is not excessive, but the humidity is high and the sky is often cloudy. Nights can be a little cool in inland areas, while during the day it can be a little cool along the coast, when the breeze blows.

What to pack

All year round, light clothes, of natural fibers, a light sweatshirt for the evening, umbrella or light raincoat during the rainy season. From June to September, you can add a sweatshirt and a light jacket, especially on the south-central part of the coast.