The contractor is responsible for it's sub contractors. You have no agreement with the tow company. Your agreement/contract is with GS. GS has a contract with the tow company and GS should deal with the tow company. You should go through the business you have a contract with. Yes the sub contractor may indemnify GS but that is their agreement you are not a party to that sub contract. GS is your primary contract provider and is the one you should deal with. What happens between GS and tow company is not your concern. That's the way it has worked for the 40 years I have been contracting. Basically GS pays you and tow pays GS and in civil or criminal cases holds harmless and defends GS against their (the tow company) misdeeds