IANAL, but my understanding is that anyone who obtains a GPL'd binary is entitled to receive the source as well, from the source that provided them with the bin

IANAL, but my understanding is that anyone who obtains a GPL'd binary is entitled to receive the source as well, from the source that provided them with the binary. If you are careful with who you distribute the binary to, you could in theory keep the source sort of "secret", though I don't think it's a good general approach to GPL software.