GOODMAN A/C UNIT GMT070-4 GAS VALUE KEPT CUTTING OFF runs about 15-20 sec. then shut off. What is the problem...

Try cleaning the flame sensor. That is a little longer than usual for a flame sensor cleaning BUT it is the cheapest first thing to try. After or before cleaning you can also test the flame sensor circuit (see attached)

Here is a complete service manual for your furnace and a few others combined. Just make sure to the service info for your model number.

http://www.hvacpartsshop.com/Gmpe075-3%20Series.pdf

Thank you veeeeeeeeery much!! That was the problem. God bless you.

Glad you got it running.