The Giphy app on Zapier only has a search, so there is not authentication needed.
You can simply search the Giphy database and we'll return a random gif from Giphy.com. When you choose Giphy as an action you can enter your chosen search term and pick a rating level for the gif if you like:
Final step here is to use the output of the Giphy search step on an Action. In this example, the url that Giphy provided is used in attaching an image to a Slack message