TEdit - Terraria Map Editor - TEdit is a stand alone, open source map editor for Terraria. It lets you edit maps just like (almost) paint! It also lets you change world settings (time, bosses downed etc), edit chests and change sign, make epic dungeons, castles, cities, and add rewards for your adventurers!
Terraria can not be started with the GOG Galaxy Client while TEdit is running, because it thinks that it is already running.
Maybe there is some easy fix by changing some window class or name or title or something.