What type of hammer and punch are you using?
Sometimes they need an extremely sharp rap to get started. The punch makes all the difference. The cheaper punches absorb the peak of the shockwave and will get you nowhere. I have had best luck with Starrett punches and so-so luck with Dasco. The advantage to Dasco is that they're about 1/10th the price and available at hardware stores. Starrett you'd have to get from a machine tool supplier, but you can find them on Amazon too.
For the hammer, 3 lb sledge is a minimum.
Get a good running start with the sledge -- it'll take confidence to bring that hammer down hard on a little punch you're holding with your bare hand!
The punch should be exactly, precisely coaxial with the pin. If it's crooked even a tiny bit, you'll lose a large portion of the striking force.j
The gas block should be supported firmly. I use the gas block bench block from Brownell's. It's not an ideal product but adequate, and works better than most workholding methods.