TSP= thrift savings plan. It's a traditional or Roth retirement account that was rolled out for service members a few years back when the retirement system was reformed from the 20yrs or nothing model. They will match your contributions dollar for dollar up to 3% and at .5 per dollar from 3-5% so that's basically free money. It's a no brainer to at least put in the 5% they'll match. G funds are one of the available investment choices within TSP. They are very low risk/low reward, so basically a bad choice for anyone who is not about to retire. They have more aggressive funds you can choose or lifecycle funds which are based on your anticipated retirement age and are a diversified mix of funds that adjust risk/aggressiveness as you get closer to retirement.