My understanding of the OP, having read and re-read it several times now, is that whilst the G string is in tune according to the tuner, when bowed along with the D (also in tune according to the tuner) the resulting interval isounds off. Also that the pitch of the bowed G string (presumably open) doesn't stay constant as you would expect, but goes flat as it's bowed.

I say it again, strings are sometimes duff straight out of the packet - DUMP THOSE STRINGS AND TRY ANOTHER SET! If that doesn't fix it, try a new battery in the tuner. If that doesn't fix it, try a different tuner.