On a giphy page copy the link to the gif from the copy link button. Add this in the classic editor by Adding Media and then insert from url. With the block editor you can add an image block and inser from url.

 

Giphy may be blocked on some school networks.

Block Editor

The gif block which requires Jetpack to be turned on, allows you to search for a gif or paste in a url.

or add an image block and insert from url. Once you have done that you can import the image usie the Add to Media library on the block toolbar:

You are responsible for any attribution needed. I’ve used the caption of the image below.