You're right about your suspicions. I'm not a professional (I suggest you talk to a real one, a one with CPA, EA or Attorney credentials and license in your State), but I would be very cautious in this case.
The IRS will look at all the facts and circumstances to make a claim, but my guess would be that the initial claim would be for this to be taxable income for your husband. He'd have to prove it to be otherwise. It does seem to be related to his performance, and I doubt that had they not known him through his employment, they'd give him such a gift. I may be wrong. So may be an IRS Revenue Officer. But I'd bet he'd think the same.
Did they give "gifts" like that to anyone else? If they did - was it to other employees or they gave similar gifts to all their friends and family? Did those who gave your husband a gift file a gift tax return? Had they paid the gift tax? Were they principles in the partnership or they were limited partners (i.e.: not the ones with authority to make any decision)? Was your husband instrumental in making their extraordinary profit, or his job was not related to the profits these people made?
These questions are inquiring about the facts and circumstances of the transaction. Based on what he can find out, and other potential information, your husband will have to decide whether he can reasonably claim that it was a gift.
Beware: unreasonable claims lead to equally unreasonable penalties and charges. IRS and your State will definitely want to know more about this transaction, its not an amount to slide under the radar.
This is not a matter where you can rely on a free opinions written by amateurs who don't know the whole story. You (or, rather, your husband) are highly encouraged to hire a paid professional - a CPA, EA (enrolled agent) or tax attorney with enough experience in fighting gift vs income characterization issues against the IRS (and the State, don't forget your State).
An experienced professional may be able to identify something in the facts and the circumstances of the situation that would lead to reducing the tax bill or shifting it to the partners, but it is not something you do on your own.