Quote from Projects/GnomeShell/Extensions/Review - GNOME Wiki! :
During the submission process, you are asked to confirm that your extension can be distributed under the terms of the GPLv2+. If your extension includes headers or a LICENSE file containing another license, such as BSD/MIT, you approving GNOME to distribute your extension under an additional license.
Can a GPLv3+ extension be uploaded to extensions.gnome.org?
My extension uses VTE, which is LGPLv3(+) (and also includes a small portion of code copied from gnome-terminal, which is GPLv3)