If you are going to research this, the name is "Gillmore" and you may get more hits with the proper spelling.
I have no personal experience with this, but found these interesting, for whatever they are worth:
https://law.justia.com/cases/california/supreme-court/3d/29/418.html
https://www.willicklawgroup.com/wp-content/uploads/2012/04/Gillmore-Gillmore-and-Trustee-Pay-over-Orders.pdf
The second one is an older blog by an attorney that references the Dunkin case and that lays out different clauses that could have been employed but that Dunkin did not use that possibly could minimize tax impact, differences in tax brackets between the parties, and specific "tax intent" language. From reading all of this, it seems to me that you should look at the actual document that your client agreed to.
I could be very wrong on this thought also, but it seems to me that this type of arrangement may fall under IRC sec 1041 that deals with transfers of property incident to divorce, and because the heart of the Gillmore decision is that one spouse can't take actions to deprive the other party to division of assets to which that other party is entitled to (by refusing to retire).
That's a long-winded way of saying I don't know. Sorry & good luck! Maybe someone else has experience that will clarify, and hope you will post what you find out.