You need a specific program to open gct, you probably ruined it when you set it to be opened with another program that couldn't open it. You'll probably have to either find a new one, or much better yet, find the raw text codes, put them in RSBE01 then use code manager to make your own gct.
I deleted the old gct. and made a new one but no matter what I do it still sets it so it opens with the program I accidentally made it open with.
Do you know how I can change it so it doesn't open with anything. Or is there something it specifically opens with?