What is a GSI?

GSI means Generic System Image, which can be flashed over any Treble Device.

So, in simple words: the same system.img can be flashed over multiple devices having Treble Support.

This happens as we now have a separate /vendor partition where all the vendor hals are stored.

Now, this will help to do easy bring ups. 

Also, if you own 2-3 treble devices, now you'll just have to download one system.img xD it'll work for all of em ( if treble supported )

How to get Treble Support?

Flashing Treble TWRP won't give you treble support. Keep that in Mind. It'll just mount the partition in the recovery, so that vendor img can be flashed.

To get Treble Support, you'll need to flash a latest ROM which has vendor image compiled with the latest sources, once you flash such a ROM, you'll get the vendor image stored in the vendor partition, and after that you can flash any GSI (as it's just System.img, and all our device specifics are in /vendor so it won't mess with that)