Use AGPL instead of GPL?
If this software is intended to be run on a server and accessed via a browser, or some other client app working over a network, you may be better to use the AGPL instead of the GPL. It's the license used by Loomio and many other free code web app projects. IANAL but my understanding is the main difference from the GPL is that AGPL includes providing users with access to a server running the software as an act of "distribution". This then triggers the copyleft provision, obliging the operators of the server to share a copy of the source code with their users and with you - including any modifications - under the same license.