When installing gnome using apt it should prompt you at some point which display manager (aka login screen) you want to use. Just choose the one that's not gdm and your login screen wont change. Generally its easy to install a new desktop. Just do "sudo apt install the_desktop_environment" and change the chosen desktop environment on your login screen when logging in (there should be some slider or button for that).

i’d like to try gnome or kde plasma

I'm surprised to see that no one has mentioned the following yet:

"KDE Edition

In continuation with what’s been done in the past, Linux Mint 18.3 will feature a KDE edition, but it will be the last release to do so.

I would like to thank Kubuntu for the amazing work they have done. The quality of Plasma 5 in Xenial made backports a necessity. The rapid pace of development upstream from the KDE project made this very challenging, yet they managed to provide a stable flow of updates for us and we were able to ship good KDE editions thanks to that. I don’t think this would have been possible without them.

KDE is a fantastic environment but it’s also a different world, one which evolves away from us and away from everything we focus on. Their apps, their ecosystem and the QT toolkit which is central there have very little in common with what we’re working on.

We’re not just shipping releases and distributing upstream software. We’re a product distribution and we see ourselves as a complete desktop operating system. We like to integrate solutions, develop what’s missing, adapt what’s not fitting perfectly, and we do a great deal of that not only around our own Cinnamon desktop environment but also thanks to cross-DE frameworks we put in place to support similar environments, such as MATE and Xfce.

When we work on tools like Xed, Blueberry, Mintlocale, the Slick Greeter, we’re developing features which benefit these 3 desktops, but unfortunately not KDE.

Users of the KDE edition represent a portion of our user base. I know from their feedback that they really enjoy it. They will be able to install KDE on top of Linux Mint 19 of course and I’m sure the Kubuntu PPA will continue to be available. They will be able to port Mint software to Kubuntu itself also, or they might want to trade a bit of stability away and move to to a bleeding edge distribution such as Arch to follow upstream KDE more closely.

Our own mission isn’t to diversify as much as possible in an effort to attract a bigger chunk of the Linux market, and it’s with a bit of sadness that we’re letting this edition go. We focus on things we do well and we love doing to get better and better at doing them. KDE is amazing but it’s not what we want to focus on.

With Linux Mint 18.3, we’ll release one more KDE edition. I wanted this announcement to come before the release. It will hurt its popularity of course, but I wanted to give users time, either to react right now or to take their time, upgrade and adapt to this later on. I’m sure this edition will be missed and I hope its users understand our decision."

From this Linux Mint blog post*.

Note that this doesn't mean that you can't use KDE Plasma (or GNOME for that matter). Though you have to be aware that you'll be on your own whenever something breaks. And if you have to ask how to change Desktop Environment in the first place, then I think that you might not be ready yet for such a ride. Instead, consider using a distro that actually does offer GNOME and/or KDE Plasma editions of its distro; the likes of Fedora, openSUSE and Pop!_OS come to mind.