Why GIFs end up sideways
Phones record video with an orientation flag rather than physically rotating the pixels. Some converters ignore that flag, so a GIF made from a portrait video can come out sideways. A 90° rotation fixes it without any quality loss. Mirroring is handy for webcam recordings (which are often mirrored), and for making a character face the other way.
If your source is a video, you can also convert it again with Video to GIF, which uses the browser’s player and respects orientation.
After rotating
Rotating by a custom angle leaves empty corners. Crop them away afterwards, or fill them with a colour that matches your page. A 90° turn swaps width and height — resize if you need a specific size. For a GIF that plays backwards rather than upside down, see Reverse GIF.