STEP 2: Get Giphy api key and Set it in GAS
STEP 3: Get Slack bot token add it to GAS property.
1. Add a Bot User
2. Enable Events (to receive POST request to GAS from slack when someone say something on slack)
3. Add permissions (the bot to send messages to slack)
Finally. Install app and set the authentication code which enables GAS to send messages to Slack.
If you want to know more about the code, please read this story.