I am having the same problem and I took a deeper look at the code. You are supposed to be able to change the first choice’s label text and that will replace the default one. However, there is a bug in 3.1.1 on line 330 of gfcptaddonbase.php . The call:
rgars( $field, 'choices/0/text/' );
contains a trailing slash, which results in an empty value always being returned for the first choice’s label and therefore always overriding your custom first choice with the default.