Both .gll and .xgll files are used by EASE SpeakerLab software to store some sort of text files. We do not know about any possible conversion between these two file types. However, it may be that you can simply resave one to another using File ► Save as.. function.
Convert EASE SpeakerLab compiled text to EASE SpeakerLab text file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert gll to xgll on Windows
A room acoustics simulation software
Convert from gll file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.