Although this is a report long time ago, I would like to clarify the situation:
src:linux produces a bunch of image and headers packages, as well as some called linux-compiler-gcc-8-x86, but these packages contain the major version of GCC as well as the architecture names which could be a burden for dkms maintainers to deal with in the long run. Also, linux-compiler-gcc-*-* only exist on x86, arm and s390 platforms and are missing on other ones officially supported by Debian. We rely on the relevant linux-headers-* packages to pull in the real compiler required by the corresponding version of kernel and in theory this dependency on gcc itself could be eliminated. Even worse, there isn’t a universal “linux-headers” package we are able to Depends on to magically make kernel headers available on any machine. But we would like to try to keep things work for most of the common cases without too much trouble for ordinary users, so it is kept in such a situation. Regards, Aron