Just an update: I received a link to download an archive that contained several source code files, however they were for the wrong modem/router model; they were meant for the TG799vac sold by Telia (Swedish ISP) instead of for the TG789vac v2 as I requested, so they are useless to me.
The build instructions were unhelpful, too. There seems to be no main Makefile, no INSTALL file, no build script or anything similar, and the only text file included just says what dependencies are needed and what toolchain to use.
I asked them to correct the mistake as soon as possible and I told them that this is not a proper way to comply with an user's GPL request.
If anyone wants to get this source code archive anyway (for example, I see that the TG799vac is sold in Australia as the "Telstra Gateway Max") please send me a private message.