Nesomyrmex angulatus is another widespread species, found from South Africa to Saudi Arabia. The closest it has previously been found to The Gambia is Ghana, so this represents another western range extension (along with Cataulacus traegaordhi). The key thing about this species that separates it from other Nesomyrmex is the very smooth profile of the mesosoma, coupled with the yellow colour.
Though I only found it twice, it's probably fairly common in more wooded parts of The Gambia. It tends to be pretty inconspicuous. I collected it from trees in Bijilo Forest Park and on Jinack Island, from coastal forest and scrub savannah respectively.