OP You have not fully stated the Rated Capacity of the pump. No way to answer your question!

Drip works ETA: see comment below.

Sorry, trying to put a link with cell phone. Never works! Ok, here it is: Dripworks Pressure Regulators. They've got a 20 psi for $8.95 and their shipping is typically fast. I'm using one of theirs and it's working quite well.

Again, without the complete pump spec, you can not tell if it will put out 20psi at whatever flow rate.

The pressure regulators above from dripworks are not typically used with a pump. You connect it to your irrigation system and your water line - water pressure from your water line is regulated via the pressure regulator.

I'm assuming you've got a pond pump and wish to save money, or perhaps you need a way to pump water from a water source with the pump. The pressure regulator might work with that as well. I know our pump is rated for a 1,000 gallon pond (I do not have all the tech info in front of me) and the pressure is not nearly as strong as coming out of a water spigot. You will want a filter, too. Irrigation lines can get clogged pretty easily if you've got 'gunk' in your water. Ours is city water, and we needed the filter.

1000 gal in 20 years or 20 seconds?

Ah, Mystic Pools is correct. OP, you've not gone into any detail, so help will be all over the place, and possibly inaccurate. My assumption, and I don't like assuming, is that you've got a water source somewhere that you'd like to use for irrigation. I also assume you do not wish to go out in person (right now, due to virus) and shop - which is why you are considering using an existing item you have at home (your pond pump). If that's the case, a cheap submersible utility pump, and a cheap pressure regulator will do for the year. Otherwise, without further explanation, I don't think anyone can be of help. Note that cheap utility pumps don't last forever, but they are cheap, whereas pond pumps are pricey.

Sorry, 1000 gal per hour