Welcome!
To add to Oussebon's comment, I do think that this PC is decent for gaming on 1080p with medium settings, considering it's 8GB of RAM instead of 8MB
.
I also think this is a good option to do if you're new into gaming and you're not sure yet if it's actually going to be used. I've adviced against buying a 2000$ gaming PC if it turns out only to be used as an office PC after 3 months. I think this is actually a precent decent start.
Unfortunately I think that not only do mass-built PC's usually use their own components, often components that should be able to get replaced can't or you may have a hard time with it. They often use very specific forms (the motherboard for this PC may look entirely different over a "normal" one. So it's not just the connections, it may be the entire layout. Or it's so small that (re)placing a component may block other components from benig used (I experienced that when I had an Acer prebuilt).
Also power supplies may or may not be an issue. I think for these kind of PC's power supplies will be designed specifically for this PC and without the need for extra connectors on them, they may be missing ones you'll need for an upgrade.
I can't really tell wether upgrades can be done or not, but I think it does require alot of research and often some tweaking too.
But I do think you'll be able to game decently on this PC for a while! Just don't get your expectations too high, you won't be able to connect it to a 4k tv and get a fluent game, and you can't expect to play the newest games on their highest settings.