That's strange. I just tested this out; yes, she does stop following you after jumping the gap, but progressing ahead a bit and giving her enough space will trigger her to jump across. Have you tried progressing ahead to the van area to see if she still follows? Are there any mod conflicts? Worst case, have you tried verifying/re-installing gmod?

Err, fixed it with the van area thing. Gave a boost with the physgun tho.

Hmmm... you shouldn't have to use the physgun to get through. Typically these sequences require a bit of space to prevent the player from colliding with the npc mid-animation.