I've seen that GET requests are size limited due to the browser not accepting excessively long urls.
But is there also a limitation in size in the response ($_GET)?
As per response in the the comments on your question. If your responses are in json format. Then they have no limitations.
See this : Is there a limit on how much JSON can hold?
And Limit on the length of the data that a webserver can return in response to a GET request
You may want to take a look at this to see how different browsers perform with large json responses how-big-is-too-big-for-json