The GL does not have a vacuum advance.
The 34 Pict3 was designed for the vacuum advance from the VW distributor. That is why everyone in the VW world has problems with that carb when they switch to the 009 distributor. Also the reason I am still rocking a vacuum advance dist on my Ghia.
First thing I was thinking about... did you block all of the Vacuum ports on the front and side of the carb? Just use those little rubber nipples from the auto parts store.
This info was taken from a page specifically about tuning the solex 34 with the 009.
"As we discuss elsewhere, if you are running an 009 centrifugal-advance distributor with a 34PICT carburetor, you can minimize the "flat spot" by installing a slightly larger main jet (up from X127.5 to X130 or X132.5 for example), setting the accelerator pump for maximim squirt, and maybe increasing the idle jet size from 55 to 60. Then set the maximum distributor advance to as much of the 28-32 degrees BTDC as it can take without the engine detonating/pinging."
The jet sizes may be different for you on the GL.
Here is another site that has some steps to try
http://www.aircooledtech.com/34pict3_modification/I am interested in hearing how it works out for you.
Edit: Can you snap some pictures of the setup installed? from a few angles would be good too.