A request to have someone stop playing jokes, to stop telling lies/fibs and to tell the truth.
Gwen wanted to hear the truth from him, so she asked him to stop pulling her leg.
The modern version of this expression conjures a sense of good humour about the whole concept, but its origins were not of good humour. The origin is found in a Scottish rhyme in which "draw" is used in the sense of "pull" rather than the word itself:
"He preached, and at last drew the auld body's leg,
Sae the Kirk got the gatherins o' our Aunty Meg."
The suggestion in the rhyme is that Aunty Meg was hung for a crime and, at the end, the preacher pulled on her legs to ensure that she was dead. The American usage suggests more of a feeling of trickery and deception.