I think you can use Ghost’s CLA: Ghost/.github/CONTRIBUTING.md at main · TryGhost/Ghost · GitHub
it is similar to Discource’s CLA, but shorter (Discourse’s CLA has a dedicated section about patents, and also a section that promises that the code will remain open source). Ghost’s CLA mentions MIT though but you can just change to GPL I suppose.
Apparently Ghost’s CLA is licensed under MIT, because in the same repo, in the readme: Ghost/README.md at main · TryGhost/Ghost · GitHub “Copyright (c) 2013-2016 Ghost Foundation - Released under the MIT license.” — which means the CLA is licensed under MIT, right? since it’s in the same repo.
Actually Discourse’s CLA and Ghost’s CLA are almost completely word-by-word identical (except for Discourse’s CLA having some additional sections). Perhaps Discourse based their CLA on Ghost’s CLA? — since Ghost’s CLA is aparently licensed under MIT. Or perhaps both Discourse and Ghost hired the same lawyers (which doesn’t seem unlikely to me). [Edit: See Sam’s comment below about jQuery (not Ghost)]