Ultimately you're just acidifying the mixture and any acid should do, so yeah. pH Down is phosphoric acid so it actually contains a plant nutrient, which is a good thing. GH pH down also has citric acid and mono ammonium phosphate, probably as buffers, I'm guessing, which would help to stabilize the pH (which is what buffers do), so you're getting a little more in the bottle than just acidification (and more P in that mono ammonium phosphate).
Do be careful with acids of course. The rule is add concentrated acid to water and never vice versa.