What did America gain in winning Guadalcanal?

When Guadalcanal was taken America and its allies prevented Japan from extending its influence across the SW Pacific. This meant Australia could be supplied with munitions and men more easily.

It also created a situation whereby Japan was forced to use up a great part of its naval resources, both ships and aircraft, to try to retake Guadalcanal.

In short; Japan had a relatively small number of high quality pilots, aircraft, and ships, but not enough for a prolonged war over a large area.