I think the free garage is only for Single Player, each character can purchase their own garage in the city which holds four cars plus there's a drop down list for any cars you've purchased on the internet. The Annis Elegy RH8 car is included in that drop-down
list if you're a social club member, plus the garage is instantly given to each character once you reach a certain point in the story.
For online I've never seen anything about getting a free garage, though you do get the Annis Elegy RH8 car, but I haven't gotten a garage yet so I have no idea when you're able to access it.
You need to buy a Garage online, then you need to open up your phone and order your free car from one of the dealerships. it is the only car there that will be listed as FREE. then it will be delivered to your garage.
Yeah, I'm pretty sure the Free Garage is only SP. I already got one for each Character (SP).
Side note, the Achievement for owning a Insured Vehicle, Garage and Apartment/ House does not Unlock when only owning a Apartment (with 2 Car Garage) and Insured Vehicle. Kinda silly why I would need to own a Garage (stand alone) even though my Apartment already
has a Garage.