I mentioned this subject in my account talk in July. This from TSP.gov.

The payment of G Fund principal and interest is guaranteed by the U.S. Government. This means that the U.S. Government will always make the required payments. In other words, your G Fund investment is not subject to credit (default) risk.
The G Fund interest rate calculation is based on the weighted average yield of all outstanding Treasury notes and bonds with 4 or more years to maturity. As a result, participants who invest in the G Fund are rewarded with a long-term rate on what is essentially a short-term security. Generally, long-term interest rates are higher than short-term rates.

https://www.tsp.gov/PDF/formspubs/tsplf14.pdf has facts about the G fund.

Both say you cannot lose principle but donít say the interest rate canít go to 0%. They just guarantee to pay the calculated rate. If it is calculated at 0%, I guess they pay 0%.