Climate map with legend:

(Please note that GURPS uses American Imperial measurements, so temperatures listed on this page are in Fahrenheit. Sorry.)

Most of the known world is much like the “American Deserts and Plateaus”; relevant excerpts from OW18:

The thin, light-coloured soil supports scattered woody scrub and cactus. The desert areas receive barely 6” of rain per year and are subject to cold winters (25°) and hot summers (90°). […] Very little rain falls here, and [most] Basin lakes are saline.

The region of the world in which our story takes place is set almost directly on the equator. That means there is very little in terms of summer/winter variation. The average temperatures of the region are shown in the image and legend above. Keep in mind that those are average temperatures; desert temperatures vary wildly due to low humidity, and temperatures in non-coastal regions can easily from 130F or more at the sun’s height to below freezing at night. These are extremes, but certainly not unheard-of.

The region is not entirely desert wasteland, of course. There is some variation due to elevation, proximity to rivers and oceans, and minor variations in rainfall. Higher regions tend to be cooler, and regions close to water tend to have higher humidity and thus more stable temperatures.

The most notable exception to the known world’s desert tendencies is the bay region to the southeast. This region is mostly sheltered by mountains, and encloses a large bay and several other major waterways. The humid winds blowing in from the ocean are trapped on three sides by the mountains, causing much higher rainfall in the region. Consequently, much of this area is humid subtropical or even swampland.