In default mode Tgastool suspends itself until the spawned program is closed and then it automatically reloads the data and activates again. This is to automate as much as possible the editing process but it does mean you have to close the paint program once you have finished editing

I you want to keep your paint program open then tick the "use old send to editor" in the prefs menu. This will switch it to the old way where Tgatool stays active and you have to manually tell it to reload the data