Is the Gas charge a one time fee, or how is that charged out on Smart Contracts?

Every function in a smart contract costs some gas amount to execute. You can find how much each statement from the opcode costs in the smart contract here: https://docs.google.com/spreadsheets/d/1n6mRqkBz3iWcOlRem_mO09GtSKEKrAsfO7Frgx18pNU/edit#gid=0 The cost to owner of the smart contract is once, at the time of deployment. Afterwards, the each user will be charged certain gas fee for execution every time they call or interact with the contract.