Stickerfy is a GIMP plugin that modifies images into proper WhatsApp' sticker standard. It scales the image to 512x512, gives them 16px wide margins, adds a solid color background that extends 8px outwards and adds a drop-shadow. It supports GIFs as well, so in the end you can export the project as a GIF (or as an animated WEBP if your sticker-pack client supports it).
Windows: %APPDATA%\GIMP\2.10\plug-ins
Nix: ~/.config/GIMP/2.10/plug-ins
With an open image on GIMP, go to Filters > Plug-ins > Stickerfy... Select the interpolation method and the sticker's color of your choice.