I would recommend that you delete the W-2G and then reenter it. Make sure when you reenter the W-2G that you don't have any extra space after the zip code because if you hit the spacebar after the zip code the software thinks that the "extra" space is part of the zip code.
How to delete W2G
Type W2G in the search box (upper right hand corner)
Click "Jump to W2G"
On the "Form W-2G Summary" page (see attached screenshot) click Delete next to the W2G that you are having this issue with.
Confirm that you want to delete it and then click "Add a W-2G" and reenter the W2G