1999 chevy tahoe getting too much gas

What makes you think you are getting to much gas? Your vehicle is computer controlled. There is an O2 sensor that monitors your exhaust and adjusts the air/fuel mixture to match, as well as a Mass Airflow sensor for how much air is coming in.

If there are any issues with either of these, or anything else with your fuel mixture, you should get a check engine light. This light means there is an error code stored in your computer that will tell you where the malfunction is happening that can be read with an OBDII scanner (available for purchase / loan / rent through auto parts stores). It could be anything from a dirty air filter, bad sensor, fouled spark plug not burning fuel, or any one of a hundred different issues.