If i buy the SR3 Full Package Gog key will i be able to use it on Steam?
I have problems downloading from Gog atmo due to my 3g dongle connection but steam gives me a reasonable download speed.

No.
As you said, it's a key for GoG, not for Steam. 2 different platforms.
They are in direct competition to each other

as neumi says they are both separate and the gog key cant be used for steam but the gog is DRM free and you can play it on your computer without the need to access the internet unless you want to play multiplayer.

Thanks guys .i thought as much .

I don't need multiplayer ,so Gog does sound better for less restrictions.