When using the stamper across multiple terrain tiles, you might run into the issue that you can't make the stamper large enough to cover all your terrain tiles. There is an internal technical limit in the stamper that restricts it from covering areas larger than 4097 heightmap pixels at the same time, which you can exceed in multi-terrain scenarios.
This article explains a workaround that allows you to place a stamp across the entire world regardless of the final world size.
Start your terrain creation process from the Gaia Manager as usual by entering your world size settings. After the terrain has been created, go to the advanced tab, and press the Terrains > Create World Designer button. This will create a "world map" that matches your world size settings in the Gaia Manager.
In the World Designer, click "stamp" first, then "flatten" (this is just to unlock all other features in the designer), then select the World Map Stamper in the hierarchy.
Delete all existing masks in the stamper and then set up the stamper to your liking. You might need to activate the preview in the stamper to be able to see better what you are setting up.
Note that this stamper acts just like the regular Gaia Stamper, only difference is it will stamp to the world map instead of your regular terrain. This means you can set up your single stamp image here like you would in the regular stamper:
When you are happy with the setup, stamp and return to the World Designer by selecting it in the scene hierarchy or clicking the "Designer" button in the Gaia Panel on top of the scene view.
In the designer, open the Export panel and export your world map to the final terrains. Note that the world designer took over your inital terrain setup from the Gaia Manager. When clicking the export button the world designer will create the terrains for you, but also copy the shape of the world map over to these freshly created terrains:
The final result is that you now have multiple terrains with the shape of the world map applied to them - meaning you have effictively worked around the heightmap restriction of the stamper: