Could you two consult and come up with a definitive answer? Thanks.
Apparently, the Georgia Code Title 21 cited above relates specifically to state and local offices, and not federal offices. Information from the GA Secretary of State says the following:
Runoff elections are held when no candidate wins the required majority of votes.
Anyone who is registered to vote in Georgia can vote in state, local, and federal runoff elections. Check your voter status.
Runoff dates for the 2020 general election are:
Dec. 1 - State and local races (voter registration deadline was Oct. 5)
Jan. 5 - Federal races (Voter registration deadline: Dec. 7)
For more information about runoff elections, contact your County Registrar’s Office.
Some caveats about my responses :
With that said, I think you can register to vote in the Runoff, but only if you were eligible to vote in the past general election but were not registered by the past deadline.
So the good news is that new people moving to GA can't vote in the runoff, the bad news is that a rush of new voters is possible.