I don't think there is a way to recover the file you uploaded back then. That file was probably resampled to smaller dimensions with 360 pixel width during the GIF generation process. I think the best way to do it would be to find that original file, re-upload it and save it as a template.
The Animated GIF Templates get stored in mp4 format at their original resolution. I haven't found an upper limit on dimensions, but there is a 5 second time limit, there may be some other file limitations for GIF template uploads.