Now you know it was not a god idèa to put glue in the threads!
(Sorry, I couldn't resist...

).
The oilfiller cap on my CS got stuck too; I figured it was because the type of plastic and oil was not stable together, the oil damaged the threads.
It's a good idèa to move it often, so when it starts to be hard to move buy a new one BEFORE it get stuck.
Oh, and buy a new one before you damage the old one...
Yes, I know it's not identical oilfiller caps for the CS and the GS, but its the same problem and the same plastic&oil, so...
-What type of oil are you using on your bike?
It say ''no syntetic'' in the manual, but my local dealer are telling me to use syntetic/mineral mix.
-Could it be the plastic does not have any problem with mineral oil? I do not know, it's only an idèa I got this wery minute, and I do not believe it my self either.
...and there is not only one way to add oil on your bike... -put it upside-down and you'll find another oil cap!

A good idèa!

Liv.