I know Gnome 3.2 doesn't have a stable version for Debian but I still would like to install it and I haven't found any instruction on their website.
Gnome 3.2 is currently in testing (Stretch). As the name implies, the "testing" version of Debian is not as stable as, eh, "stable"!
That said, it is definitely a lot safer than running Debian unstable. The easiest way for you to install Gnome 3.2 would probably be to switch to testing. Or...
You could mix your repositories, which I personally am not a fan of (can cause odd and unexpected behavior and migraine). But apparently not all share my concerns. That said; don't do this in a server. Just don't. But since you are looking for a desktop environment, I am guessing this is for a desktop computer.
From the official Debian FAQ:
3.1 Which Debian distribution (stable/testing/unstable) is better for me?
The answer is a bit complicated. It really depends on what you intend to do. One solution would be to ask a friend who runs Debian. But that does not mean that you cannot make an independent decision. In fact, you should be able to decide once you complete reading this chapter.
If security or stability are at all important for you: install stable. period. This is the most preferred way.
If you are a new user installing to a desktop machine, start with stable. Some of the software is quite old, but it's the least buggy environment to work in. You can easily switch to the more modern unstable (or testing) once you are a little more confident.
If you are a desktop user with a lot of experience in the operating system and do not mind facing the odd bug now and then, or even full system breakage, use unstable. It has all the latest and greatest software, and bugs are usually fixed swiftly.
If you are running a server, especially one that has strong stability requirements or is exposed to the Internet, install stable. This is by far the strongest and safest choice.
Rather than worry about Debian 8, 9.0 (Stretch) is just around the corner and is quite stable - It's my daily driver, and I'll be keeping it for a year anyway when it goes stable. Stretch has Gnome 3.22.2 and it's rock solid for me (since it's in freeze).