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.
Source: TSP: Plan News and Announcements