Are you sure you don't have it? I mean, that it's not in someone else's inventory, or over in the "quick use" slots?
If it's not, one of three things may have happened.
1. You have already used the key, and it's gone. Sometimes you don't notice when something gets used.
2. You dropped it someplace. Good news is that items like that tend not to disappear. Bad news is that you may have to cover a lot of the Gael Serran to try to find where you left it. The Reveal spell can be a big help with this.
3. You've run into some other strange game bug... and it may be necessary to hack the key for you. Wyvern has a post way back about hacking the game... use the search feature and you can find it. I don't remember if the code for the key is in that post (and its follow-ups), but I'm sure it can be found.
If all else fails, though, you may have to go back to a prior save game.
