The gx1 file should be placed in the Heroes2/Games directory. After that, launch the game, and this save will appear in the Games menu.
The possibilities in the second Heroes game are quite extensive.
I've checked, and it's indeed Len's mission. But I'm still curious about how he managed to defeat the Necromancer at the beginning of the game, considering they start with 40 skeletons and the Haste spell.