General: PNG must be saved in indexed mode.
GIMP: Import the .gpl as your palette and enable alpha layer. Functionally pixels with transparency will become indexed to #0.
Aseprite: Import the .png as your palette, the magenta in slot #0 will be treated as transparent while in indexed mode by default.
Note: The colors of your palette don't actually matter, we only care about the index number. Below is the default colors and how they're converted.