How to restore gradient maps from old photoshop version?

Another thing: Did you update or upgrade? 

Do you have some back-up system in place? 

Just to make sure: Did you ever actually save your custom presets? 

A good resource for the many types of Photoshop settings files is this document:

Preference file functions, names, locations | Photoshop

If you use the page search function of your web browser to look on that page for “brushes” and “gradients”, you will find the folder paths for the brushes.psp and gradients.psp settings files. In your case, be sure to use the path listed for Windows, because that page also lists them for macOS.

After you learn the folder path for the settings files you need, you just open a computer backup from before the Photoshop update was done, and restore from that same path.

In the future, when you update/upgrade, be sure you select the option to import previous settings and preferences (see below), so that the settings you were using will be carried forward to the new version. If that box is not selected, your settings get wiped. If you have set the Creative Cloud desktop app to update automatically, the option shown below is in Preferences > Apps > Auto-Update, in the Creative Cloud desktop app.