I couldn't navigate into C:\Program Files\WindowsApps to find the gimp-3.exe because it says access denied.

To add the Windows Store version of GIMP 3 as an external editor you need to find the path to gimp-3.exe in the Registry

In Registry Editor

Search the HKEY_CLASSES_ROOT registry area for "GIMP"

The Registry Editor should open to a folder tree that points to GIMP

Expand and take note of the (Default) value under HKEY_CLASSES_ROOT\AppX38259ydx20yaj0afsxhqwr6xjp2ks4nj\Shell\open\command\

"C:\Program Files\WindowsApps\GIMP.43237F745459_3.0.60.0_x64__nq49gba4h4mx8\VFS\ProgramFilesX64\GIMP\bin\gimp-3.exe"

In IrfanView

In IrfanView go to Options => Properties => Miscellaneous find the 'Set external editors:' fields and enter "C:\Program Files\WindowsApps\GIMP.43237F745459_3.0.60.0_x64__nq49gba4h4mx8\VFS\ProgramFilesX64\GIMP\bin\gimp-3.exe" (without the quotes) in one of them.

Click OK