First, replace the core GNOME apps you use most frequently with lighter alternatives. Take it easy, do it one at a time. Try rxvt-unicode instead of gnome-terminal. Try Thunar or Dolphin or an mc clone instead of Nautilus.
There's a lot of non-obvious configuration options in the X11 stack. An .Xresources file can select better fonts, tweak hinting so they look fantastic, and change settings and color themes for many X applications. Out of the box, many X applications look like obsolete garbage, but they can be lightweight, functional, and awesome looking at the same time with a little configuration and love.
After getting your core apps sorted, consider an alternate DE/WM. I personally love the gaps fork of i3. XFCE is nice if you still want DE functionality.
I personally dual boot Debian Stretch and Gentoo ~amd64 - sharing a home partition. I keep Debian more or less stock on GNOME, while I rice the hell out of Gentoo with i3 and no systemd.
All you need to do is install XFCE. It has a great terminal, a solid file manager, a compositing WM, and easy configuration. I don't bother setting things up piecemeal or fiddling with .Xresources any more.
Also, it would be great if we could stop using "rice" as an adjective. Even if you don't find it offensive, some people do.
Contrary to some other advice here, I switched to a new window manager first (xmonad, FWIW), but kept running a bunch of GNOME stuff too. Gradually, I have phased out most of GNOME and my computing life is simpler and more stable as a result.
