Per the write up in the LA Time:
According to a person familiar with those talks, Gates, a longtime political consultant, can expect "a substantial reduction in his sentence'' if he fully cooperates with the investigation. He said that Gates is apt to serve about 18 months in prison.
The delicate terms reached by the opposing lawyers, he said, will not be specified in writing: Gates "understands that the government may move to reduce his sentence if he substantially cooperates – but it won't be spelled out.''
Emphasis is mine.
To summarize: Gates is prepared to testify against his former employers (being most directly Paul Manafort, but by extension Donald J Trump) while still entering a guilty plea with the Courts, and more importantly with no written guarantee his cooperation will result in a reduction in his possible sentence!
I humbly suggest that Gates is seriously desperate to roll over on everyone possible he’s willing to forego written guarantees there. Its equally plausible the Special Counsel is not sufficiently convinced of either Gates’ sincerity, or possibly of the value of the treasure (to use IC parlance) he’s offering, that they need the extra stick of not putting anything into writing.
Given who we’re talking about, I suspect its more the former than the latter; plus which I can’t see Mueller signing off on something like if he wasn’t confident it would move his larger investigation forward.
It needs to be re-iterated that Gates was both the Treasurer of the Trump Campaign through most of its critical points, and was present and involved in the actual Transition. This means he knows as much, if not more, about the workings of both the campaign and transition as Manafort. It wouldn’t be a stretch to think he’s in the know about the sources of the campaign’s funding, including where the money went, and be able to account for the operations/contacts/negotiations that went on after Election Day.
In short: if this plays out the way it looks, Gates might be handing Mueller the keys to the proverbial kingdom. Gates has strong incentive here to give his fullest cooperation and hold nothing back.
We’ll have to see what happens next. Given the stakes and the likely targets, we should be prepared for literally anything.