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Porn Gifs (not videos) lasting longer than 15 seconds.
LONG gifs (>15 seconds) can be posted to gfycat if they are uploaded in gif format, not video format.
TIP: Gfycat limits uploads to 15 seconds if you're using the web interface BUT video files up to 1 min can be uploaded directly to gfycat via the Gif Brewery 3 Mac app (http://gifbrewery.com/). It's free (no upsell, ads or crippleware - gfycat acquired the app and it's to their benefit if you use it and their service).
The other way to upload to gfycat is to convert your video file to gif format first -- the restriction is 300mb, no time limit.