instructions

You need Map Editor by Guadmaz https://it.gta5-mods.com/scripts/map-editor, 

If you just want to load maps
F7 -> load -> xml -> franklin_garage3 or franklin_garage3_empty. (empty is without a car)


If you want the SPG version as well then you need Single Player Garage by mlgthatsme https://it.gta5-mods.com/scripts/single-player-garage. 
First copy the folder "Franklin_garage_3" in scripts/SinglePlayerGarage.

Then load the map whit Map Editor, you can put it in the scripts/AutoloadMaps folder to load it on startup.
F7 -> load -> xml -> franklin_garage3_spg

Restart the game

Enjoy