I am building a News App.Is it ok to use Google news rss feed for my app.Is it free or they charge for it?
I suggest you take a look here at Google's ToS. That should give you an idea of what you are / are not allowed to do.
However, I would advise you to check with an attorney or someone else who is qualified to give you legal advice.
Edit: On a closer look at the Google News ToS here it looks like you may not use this service for commercial reasons.
Again, you should check with an attorney because it looks like Google has changed their terms a few times and their latest ToS page (March 2020) does not refer to the specific Google News Terms of Service I linked earlier (which is from 2006).