Hello,
It is possible that the photos are still syncing, which may cause the Gallery images to take some time to load. Please wait one or two hours and check whether the issue resolves itself. If the problem persists, restart your Windows PC as a first step and check again.
If the issue continues, open an elevated Command Prompt, enter the command below, and press Enter.
Sfc /scannow
Once the command has finished running, restart your computer and check whether Gallery responds correctly.