GNOME 3.20 is only available in Debian testing, so you can:
- update to testing (which isn't recommended as it isn't stable)
- install only gnome from testing (which can create new problems as gnome has many dependencies and mixing releases isn't recommended (see Debian Wiki)
- use a different distribution which is using a newer GNOME version.
- try to find out if you really need GNOME 3.20 or you can resolve your problems with 3.14
- wait until the current debian testing (stretch) becomes stable