How to repair a jacket whose pocket zip has detached from shell?

Sew it back on and tape the seam.

Stitching GoreTEX is the same as stitching any other material, the difference is you want to use Tenara thread, and you waterproof the seams on the inside of the garment after by ironing on GoreTEX seam tape. You can get any seamstress to repair your zipper, they'll probably tape your seam for you as well. If you want to do it yourself, you can purchase seam tape from multiple sites online including ebay and amazon.

I have found that many drycleaning/laundry agencies (where you drop off your clothes and pick them up later, not laundromats) will do small repairs/alterations.