Pasting the Reels share link, you would be directing the Gif Maker to the webpage and not a video.
You would first need to save the video you want to use in the Gif Maker.
It means Imgflip has support for YouTube. And Imgflip does not have support for Instagram Reels, which means you have to get the direct video link, which usually means you have to download the video.
TikToks work fine.