Typically there is a primary target platform, which will be Steam because of it's market size. Everything else I would expect to be secondary; how much attention is paid to those platforms is likely driven by their market size and how much they diverge from the primary platform.
GOG, although a much smaller market, is very similar to Steam, in that the base game is on the same OS and graphics API, with MP features either dropped or replaced with Galaxy, so it shouldn't take too long. Stadia, on the other hand, is Linux and Vulkan, so it will probably be a post-release port, unless Google is offering incentives.
