It really does depend what he did, and whether he can get ahead of it.
Gross misconduct, i.e. instant dismissal, is very, very serious. Considering how litigious employment law can be, the fact that your DH has just accepted it without a fight is also indicative of its severity.
Our Employee handbook says GM is "any behaviour or negligence resulting in a fundamental breach of contractual terms that irrevocably destroys the trust and confidence necessary to continue the employment relationship"
Examples include theft and fraud, physical violence and bullying, deliberate damage to property, deliberate acts of unlawful discrimination or harrassment, possession of or being under the influence of drugs or alcohol, breach of H&S rules that endangers lives, undertaking private work on site, working in competition, doing something that results in adverse publicity, breach of confidentiality.
If, for example, he'd been stealing money or equipment, I think he'd be hard pressed to defend himself with future employers.
If, however, he'd got a side hussle to earn some extra money and got caught doing it at work, it would be a bit easier to soften it at an interview.
If it is true that there is a lot of gossip, the best thing would be to just nip it in the bud and explain what happened. People may think badly of your DH, which could potentially be justified, but at least it would stop the rumour mill.