Since you got that used there is a chance that it has the older spring style pressure valve, which you can just adjust yourself. I did it to the used one I bought years ago, it was very easy.
https://www.blackoutcoffee.com/blogs/the-reading-room/gaggia-classic-opv-adjustment
If you have the model I'm talking about it isn't even an upgrade, it's just an adjustment. There's tools you need if you want it to be perfectly exact, but eyeballing it worked fine for me.
The newer ones you have to buy a replacement part though.