You don't have to install it.
Just get the free copy on Epic Games and just download it whenever you're good.
But too bad that the free just expired.
I see, I've claimed it like several days ago.
Because there's also a deadline for the 1 million bucks head start for GTA V online by accessing the server for the first time, just not sure when.
It seems the offer ends by today (more precisely by May 21). 
Yes, it expired but there's a new free game from Epic games again and there will be another batch that's on the mystery list.