I can confirm that add-on won’t update on Xammp installation. You have to do it manually.
1. log in to gp go to your account. Make a copy of your key number and save it in notepad.
2. Also in your account you will find the link to download a copy of the add ons. Click on the link and download the zip file and then save somewhere on your desktop.
3. Start your website in Xampp. Log in to admin. Go to plugins and select upload new, browse to the location you saved the add ons zip on your desktop. And upload. Then click on activate.
4. Then go to apearance, generatepress, and check that your addons are activated. If not activate them.
5. Then copy the key you saved in notepad and paste it in the update key field to the right of the addons screen. Click on the update button and you should get the green updated tick.
Every time Tom releases a new update to the addons you will have to deactive and then delete the addons plugin. Then go back to the gp website and download the latest version using the link in your account. And then repeat the above process.
Hope that helps.