Is the gerber stem recessed in the wall? If it is, you probably cannot
see if there is another nut you have to loosen inside the wall.
I had a similiar situation like yours. I was pulling the stem with a
lot of force and it did not come out. Finally I decided to take a look
inside the wall and sure enough there was another nut I had to loosen.
You need a long deep rachet to fit over the stem to remove it. Of
course with so many brand faucets and stems out there it's tough to say
if you have the same one as mine. Good luck.