I could save 3 bytes if I could require the caller to pass the chroma-key constant (0x00ff00) as an extra arg, instead of hard-coding the constant into the function. Chroma-key tool controls The chroma key tool constructs a mask, shown in the Key channel graphic on the left side of the tool, where the transparent part of the frame is drawn in black, and the opaque part the part you will see in the final video is drawn in white. By using this service, you agree not to hold ImageMagick Studio LLC liable for any data loss, subsequent damages, or privacy issues resulting from the use of this service.
The above is not an exhaustive list by any means, they are just the first few things that came to mind when I thought about the differences. I didn't even mention support for HEIC (Apple's format for iPhone images), increasingly common High Dynamic Range formats such as EXR, or any others. In fact, if you compare the file formats supported by the two products (gm convert -list format and magick identify -list format) you will find that IM supports 261 formats and GM supports 192.