Does any TSP fund guarantee a rate of return?
No. The only assurance is that the most conservative fund, the government securities G Fund, will produce a positive return. Because of the way the fund is invested, it cannot suffer a loss. However, no particular rate of return is guaranteed. It roughly reflects the returns of mid-term government bonds.

The returns of the other funds mainly reflect gains and losses in the stock and bond markets whose returns they track; trading costs, interest and dividends also are reflected. The international stock I Fund further reflects changing values of foreign currencies versus the U.S. dollar. The lifecycle L Funds reflect all of those changes since they are made up of differing mixes of the five underlying TSP funds.