"Declassified documents show security assurances against NATO expansion to Soviet leaders from Baker, Bush, Genscher, Kohl, Gates, Mitterrand, Thatcher, Hurd, Major, and Woerner" [0]
That isn't a counter to a primary source that says he never discussed it.
Although I do know that NATO Expansion is another historical grievance Russia has (getting lots of these aren't they) because Russia is very much pro being able to invade states, and NATO stops that.
These documents are primary sources. Gorbachev says contradictory things.
"Soviet President Mikhail Gorbachev, who participated in the 1990 negotiations, subsequently spoke out about the existence of a "guarantee of non-expansion of NATO to the east" inconsistently, confirming its existence in some interviews and refuting in others." [0]
Here is what Gorbachev was saying in 2008.
"Relations have further deteriorated after Nato promised eventual membership to Georgia and Ukraine, a move interpreted by Mr Gorbachev as an attempt to extend America's sphere of influence into Russia's backyard.
"The Americans promised that Nato wouldn't move beyond the boundaries of Germany after the Cold War but now half of central and eastern Europe are members, so what happened to their promises? It shows they cannot be trusted."" [1]
[0] https://nsarchive.gwu.edu/briefing-book/russia-programs/2017...