Do Geophysical Research Letters (AGU) accept PhD thesis as reference? Do Geophysical Research Letters (AGU) accept PhD thesis as reference?
In other words, can be possible to cite a PhD thesis in a GRl paper?

On their website they stated that any unpublished studies will not be accepted.
Of course. At all universities I've ever heard of, the PhD thesis is public (potentially after a waiting period for intellectual property reasons), and consequently it can be cited.

It may of course be that these theses are not always easy to get -- they may not be on the internet, for example -- but they exist and are available to the public from the library, for example.