This works using Gaia 2021 & Gaia Pro 2021:
This method works for those that need to resize their world or add more terrains to their world.
This first example we will use a large world 2048x2048.
We are going to add more terrains and add streaming.
First we will want to add the terrain scanner: Window - Procedural Worlds - Gaia - Show Gaia Manger - Advanced - Tools - Scanner:
Then drag your terrain into the terrain scanner:
Then save your scan. This will create a heightmap that we can then use in the stamper later.
When the heightmap is saved we can then delete the scanner out of the Hierarchy.
Next we can delete out terrain and go to the Gaia Manager again.
We will go to step 2 - create world and under the world size we will hit the (+) icon:
Next for this example I am going to create a 3x3 world at 512x512:
Next I am going to change the Workflow from World Designer to Manual with Stamper.
After the Stamper is added we are going to change the drop down to Scanner Exports and select our terrain that we scanned.
Now you can see your original terrain and you can stamp this in.
If your height values are not correct you can use the scale tool (R Key) and change the Y value or Yellow arrow:
This will change your height if you needed to add more height to the world.
Then stamp your terrain.
If you have a multi-terrain setup already but you are wanting to add more terrains the process is slightly different.
You can still do the process above but depending on how many terrains you have this might take some time.
Taking the example above and working off of it.
After that multi-world / multi-scene is created we could add more terrains by going back to the Gaia Manager - 2.Create Worlds - open the advanced section.
You will be able to change the tile size again to 4x4 or what ever you need.
Just make sure that Re-center on update is enabled.
This will take your current terrain setup and add more terrains surround your existing world.