GNOME

Use the NetInstall (https://trisquel.info/en/download has a link behind "More") and execute 'sudo apt install gnome': https://trisquel.info/en/wiki/installing-gnome-shell

> How do I get the best, most complete GNOME experience on Trisquel?

I'd start with a netinstall, so that you don't have any MATE stuff
cluttering your system.

To install GNOME:
$ sudo apt install gnome

For Trisquel look/feel (Trisquel's icon theme doesn't play nice with
GTK3, but GNOME Brave looks very similar. You'll need gnome-tweak-tool
to change the theme.):
$ sudo apt trisquel-gtk-theme gnome-brave-icon-theme gnome-tweak tool

Totem is buggy.
$ sudo apt remove totem && sudo apt install mpv # or vlc

Unless you really want to use Epiphany,
$ sudo apt remove epiphany* && sudo apt install abrowser # or icecat

If you want graphical package management tools (GNOME software is buggy)
$ sudo apt install gnome-app-install synaptic

If you have a touchpad that supports multitouch gestures, you could
replace the synaptics touchpad driver with libinput and build
libinput-gestures so that you can 3-finger swipe to change workspaces
and 3-finger pinch to open the dash. IIRC you have an X200 though, so
you might not have a touchpad.

How do I get the best, most complete GNOME experience on Trisquel?
well the closest one is the best..Trisquel has good gnome already...

try and read about it. Mr,banana has pretty good instructions