If you are doing a one off piece of work as a consultant, for which the client receives only binaries, can you use gpl3 software ? You are working on behalf of the client and not distributing it.
From a standard gpl FAQ:
Q: I am an OXID partner/reseller/consultant and I occasionally do one-time OXID eShop Community Edition customization work for clients which are derived from OXID modules. These customizations are used only for the client. Do I need to make these changes publicly available? A: No, unless your client redistributes the code in which case the client must make it available to its licensees.
Is this implying it is ok?