A common approach to optimising you game is to use a streaming tool like World Streamer to stream in small terrain chunks. 

However, Gaia 1.x can only work on one terrain at a time so another approach is to make one LARGE terrain with Gaia and then split it up for use by World Streamer.

A great and free tool to do the splitting is here : http://kostiantyn-dvornik.blogspot.com/2013/12/unity-split-terrain-script.html