You can find it in the "Local Settings" section in the settings. If you are on mobile, you have to scroll down from the settings block. Also you may need to clear caches.
Note:
These options are kept on the device and are not synchronized between devices.
Is there anything that can be done to have RedGifs thumbnails show up like other images/gifs? A proper embedding of those links would be an amazing improvement, lots of porn to share from there :)
Stupid Redgifs is now blocking direct access to even thumbnail links. I wrote a simple proxy script to get over it and its working. But I can't rebuild this version right now. It should be available in 0.19.4