For some reason, the existing Giphy app for slack does not allow bots to call a GIF from giphy.com
If I type \"/Giphy sad\", the Giphy app will automatically load a sad
The official API does not support invoking other slash commands, but there is an unofficial API method that does it:chat.command.
Note that this will only work in situations where you have full control over a Slack workspace and can use legacy tokens.