There is an opening threaded metal pipe in the tank near the gas master control which can be inserted by a faucet where the water is coming out dripping in an estimated volume of (1) liter in about 3-4 hours. (Ameri-Glass Water Heater Model #GVF-8433T SN-8934311195, 40 gal capacity.)

Can I possibly insert a faucet to close the opening and lock the faucet to stop the dripping?

you could I guess,but if or when the water tank blows up and possibly does a bunch of damage to your home (maybe even burn it down) your insurance company will be asking a bunch of questions
I would replace the pop-off valve or find out if maybe the temp. setting is set to high

this I think is what you are talking about,,,,its a safty pop-off to keep the tank from blowing up