Without me doing further research, you can't ban or unban by IP address using Gban.
Per the usage instruction in the first post of UBan, you can only unban by steamid.
Usage:
ulx gban (user) (time) (comment) -- Ban a user by name in the server. Time is in minutes, use 0 for perma.
ie: ulx gban Megiddo 0 "Dirty spammer"
ulx gbanid (steamid) (time) (comment)
ulx gunbanid (steamid)