I'm on my PC, so if you're doing it on your phone it might not work. But regardless, try this:

1. Right click over the gif and select "Copy Image Address"
2. Click the Insert Image Icon in the Quick Reply toolbar above your post [the square between the globes and the film strip]
3. Select "From URL"
4. Paste your image address in the space
5. Click out of the box beside Retrieve remote file and reference locally. LEAVE UNCHECKED.
6. Select OK