That seems more like a Slack setup issue than an issue with Slack itself, to be honest. We have a separate #random channel for stuff like that and have turned off notifications for it. It's still there, but it's much less intrusive.

You might also want to check out the settings in the Messages & Media section, specifically "Expand links to images, video and audio from external sources". That might help to disable the inline GIFs, although I haven't tried it myself.