Dont cache hook_form_form_id_alter I made a form, with a few fields. In the hook form alter I add some info text depending on the value of a $_GET variable. 
Only the hook alter is only called the first time. 
I want is to be called every time the form is viewed.

Am I using the wrong hook?
You need to either disable cache entirely with max-age = 0 or to make it vary based on the GET argument value, which is the better way unless your GET argument unique for everyone or most people.

Something like $form['#cache']['max-age'] = 0;

See https://www.drupal.org/developing/api/8/render/arrays/cacheability for the documentation.