i have downloaded a starter edition of a theme which is under GNU GPL 3.0 . but many functions of this theme is disabled and i have to pay for these options to work. but when i changed some lines of codes these functions start working. my question is that can i use this modify theme on my website without paying for that because the starter edition which i downloaded is under GNU GPL License.
It's stealing. So no one can counsel you on ethics. You are going to do what you want no matter what we say. Most honest webmasters would just pay for the theme, or wait until they have the money to do so. I think it's pretty shitty to take what the designer so generously provided for free use and look for ways to bet him out of his work, but, I can't make that call for you.