gct is a command-line application that just makes a GCT from a .TXT file, using only lines of this format (where X is a hex digit):
* XXXXXXXX XXXXXXXXor
The first argument must be a .txt file. (On Windows, you can drag-and-drop the .txt file onto the .exe.)
If there is a second argument, it will be used as the output path for the .gct. Otherwise, the program will try to write to one of the following files, in this order, where [gameid] is the first line in the file having 4 letters/numbers followed by 2 numbers.
At least on Windows, the program will not stay open to display any success or error messages. Check the last-modified time of [gameid].gct to see if it worked or not.
With the 2015-12-21 update, .txt files encoded in UTF-16 (big or little endian) are now supported.