After installing the app, I can't make calls over GSM anymore
Usually it happens because other programs for IP calls installed on your phone interfere.
Here is what you can do. When making a call, the first menu you'll see offers you to choose a program for the call - Zadarma or another IP calls client. Choose Zadarma, and in the Zadarma app choose to call on GSM.
If you don't want to make a choice in two menus every time you make a call, check in the first menu (which is not related to Zadarma app) a mark «Remember choice».
You can also uninstall previously installed programs.