Just got an email from the TSP about the G-Fund:
G Fund and the debt limit
As of today, March 16, 2017, the U.S. Treasury was unable to fully invest the Government Securities Investment (G) Fund due to the statutory ceiling on the federal debt. However, G Fund investors remain fully protected and G Fund earnings are fully guaranteed by the federal government. This statutory guarantee has effectively protected G Fund investors many times over the past 25 years. G Fund account balances will continue to accrue earnings and will be updated each business day, and loans and withdrawals will be unaffected.
This seems to happen every year or two.