If the target land is an illegal target by the time Geomancer’s Gambit tries to resolve, the spell doesn’t resolve. Its controller doesn’t search for a basic land card and you don’t draw a card. If the target is legal but not destroyed (most likely because it has indestructible), its controller does search and you do draw.
Destroy target land. Its controller may search their library for a basic land card, put it onto the battlefield, then shuffle.
