I hate to tell you but you quite likely destroyed that combination gas valve on the furnace.
No combination gas valve that I am aware of will withstand an inlet pressure of 15psi. The valves are designated as having a MAXIMUM inlet pressure of 1/2 psi.
It appears that you are using some salvaged fittings, that's a no-no. And why are you using a gate valve? Only approved AGA (American Gas Association) valves should be used in gas piping.