Am I required to pay Gross Receipts Tax if I am a 501C3 organization?

Taxation of 501c organizations is a specialized area of taxation, so your average tax professional will NOT be able to answer your question. Sorry!

Gross receipts taxes are state taxes, and rules vary widely from state-to-state. So: it depends - what state are you in, and what types of products or services are you selling that falls under a gross receipts tax in your state?

Do you give your tax exempt ID when you make purchases?