This colored output is a native feature of Scite editor (GUSEK is a Scite fork).
Unfortunately, didn't fine any help to point to you.
There are based on output pattern (normally at beginning of line):
Normal line ! = Red line > = Blue line - = Orange line + = Green line
(<N>) : = Red line (jump to line N when double-clicked) Start with String or Integer then <TAB><N> = Pink (jump to line N when double-clicked) <filename>:<N>:<message> = Dark Pink (jump to line N when double-clicked)
Where '<N>' is a number, '<TAB>' is the tabulation character, '<filename>' is the model file name and so on.
The last line grants that glpsol error outputs can be double-clicked to select error on model files. Eventually I use it to put some highlight on printf outputs.