I asked my husband if he would take a GABA supplement to improve his mood

How does this..

he responded by saying he'd contact the attorney for divorce papers.

Warrant this kind of response?

and even though he'd never hurt me, he can intimidate very well.

The fact that you even feel the need to bring this up is a huge red flag.

He feels I want to change his behavior with an unnatural substance

He needs to be more educated if he really thinks that GABA isn't naturally produced by the body anyway. You've really done nothing wrong, sweetheart.

Your husband is verbally abusive and the fact that he can't talk to you like an adult and immediately threatens divorce is screaming that this is an unhealthy and toxic relationship. The fact that you find your husband intimidating is incredibly terrifying from a bystander's perspective.

My SO is a big hulk of a guy, 6'5", 245 lbs, and I have never once been intimidated by him despite being a foot shorter and 110 lbs lighter than him. Never, ever, ever, ever have I been afraid of my SO for any reason whatsoever. And he's a strong willed individual with staunch opinions and a big voice, but he doesn't intimidate me because he sees ZERO reason to exude any type of power, mental or physical, over me. He sees me as his equal. The fact that your husband is intimidating you on purpose (i.e. he is actively doing things he knows makes you uncomfortable) is very unsettling.