Here’s how to install gradients (GRD files) in Photoshop: 

1. For Photoshop CC 2020 and newer: Open the Gradients Panel (Window > Gradients). Click the menu on the top right corner, then click Import Gradients... Select the file ending in .GRD, and click Open.
2. For Photoshop CC 2019 and older: Go to Edit > Preset > Preset Manager… then select the Gradients preset type. Click Load… then locate the file(s) ending in .GRD, and click open. 
3. Your patterns will be available to use with your Gradient tool, the Gradient panel, or where gradients are found in Layer Styles (Window > Tools or Layer > Layer Styles).

Here’s how to install gradients (AI files) in Illustrator:

1. In Illustrator, open up your Swatches Panel (Window > Swatches)
2. Click the menu button in the top right corner of the Swatches panel.
3. Go to Open Swatch Library > Other Library...
4. Locate the gradient (.ai) file library, and click open. 
5. Your gradients will appear as a separate swatch panel. 
6. Any swatches you select and use will appear in your main Swatches Panel also.