Just got Xenonauts on sale. 75% off! Wondering about installing. It lists Xenonauts (Part 1&2) as having already patch 1.65 (gog-13). Then why include Patch 1.59 and 1.65 separately further down? What is the best way to install this game?

They are there for those who installed the game with one of the earlier installers. Best way is to use the, already updated to 1.65, installer (save both parts in the same directory). You don't need, nor can you use, either of the patches as they are already applied (skip them).