3
Gzip compression is similiar to a zip file, what it does is compresses a file on server side and just before it hits your browser it gets de-compressed.
Good point about it is that files appear quicker in your browser, but the bad sides are you cannot have more than one script (Xoops and IBF module) using it and it puts an extra (slight) load on your server.
But personally, I think the good points outway the bad.