This is actually the legal definition of a contract. BOTH parties have to gain something. If I get you to sign a contract that says you will never speak negativ

This is actually the legal definition of a contract. BOTH parties have to gain something. If I get you to sign a contract that says you will never speak negatively about me, but I don't give you anything in return, the contract is null and void. I believe this is referred to as "consideration." If there is no consideration (i.e., benefit) for you to sign the contract, then it can't be enforced. That's why if someone asks you to sign it, you can either ignore it or demand some sort of consideration (e.g., compensation) in return for signing it. Otherwise, it's unenforceable.