The likely scenario is that, as described, his cutting off from his powers and the massive reduction of his capabilities happened when he discovered that cut off section of shadow weave, and as a result he literally couldn't get himself back to the material plane at all in the condition he found himself in – that he has not, in fact, been back to Waterdeep since acquiring the orb, and the ex-planar pocket he found it in may have afforded him other options to get out, but not sufficient to get him home before he was picked up by plane-hopping Ilithids – Or even that it was those very Illithids that found him stranded in that pocket and took him there, since we know that the illithids have been working with netherese magic, which is related to his immediate situation and the shadow weave chunk, and the pocket he found it in.
gale's power is banned by mystra, this is gale personally says to the player character.
mystra doesn't only abandon gale, but also ban gale use the advanced magic weave, this is the real reason that gale lost his magic power.
