Based on expansion votes in the ‘90s and after, it appears that a 75% majority is needed for decisions like this.
You don’t want it unanimous, since one lonely hold-out would hold up things, and a simple majority could leave too many schools out to dry.
The GOR agreement itself purports to require unanimity to amend or modify it (which would include amending it by terminating it), stating in Paragraph 8: “This Agreement may not be modified or amended other than by an agreement in writing signed by duly authorized representatives of the Conference and
each of the Member Institutions that are then members of the Conference.” It's no surprise that you would try to require unanimity on this since terminating the GOR would immediately put the league in breach of its contract with ESPN.
None of the provisions in the ACC's Constitution or Bylaws speak specifically to the issue of what kind of vote is required to terminate the GOR (for much the same reason -- since it would immediately breach the ESPN contract, there would be no incentive to spell out how to do that). The Constitution and/or Bylaws do spell out differing vote requirements for different things that come close to the GOR issue but don't directly hit it.
For example, as DU82 notes, you need a 3/4 vote to admit or expel members (Constitution, sections 1.4.3. and 1.4.4).
And, there is a 2/3 vote requirement for "entering into or amending any Material Media Rights Agreement" (Constitution, section 1.6.2). Obviously, the ESPN contract is a "Material Media Rights Agreement," so would require a 2/3 vote to change. But, the GOR itself probably doesn't meet the definition of "Material Media Rights Agreement."
The only other avenue to terminate the GOR would potentially be to dissolve the conference. No specific provision in the ACC Constitution states what sort of vote is required to dissolve the league, and the ones that get closest are arguably conflicting. Section 1.6.2 of the Constitution states that "[e]xcept as otherwise provided herein or in the Bylaws, ... the affirmative vote of a majority ... shall be an act of the Board." But, that section also states that a 3/4 vote is required for "any amendment" of the Constitution or Bylaws.
My guess is that, if litigated, this would be interpreted to allow dissolution of the conference based on a pure majority vote (as dissolution really is different than "amendment," even though it's odd to allow the much bigger change to be carried out by a reduced vote). But, it's ambiguous. So, adding Stanford, Cal and SMU was fairly clearly aimed (at least in part) at protecting against the risk that the "group of 7" schools who were reportedly seriously considering doing this (Clemson, FSU, Miami, UNC, NC St., Virginia and Va. Tech) could flip an 8th school (like Georgia Tech) to vote for it.
And, of course, all of this ignores that even if you can dissolve the league to get out of the GOR, that would also breach the ESPN agreement. I suppose the consequence of that would likely just be exposure for a damages award and it wouldn't literally prevent the BIG, SEC or Big 12from adding the leavers; it's unlikely a court would order specific performance of the ESPN agreement, but it would be humorous if, for example, the leavers voted to dissolve the ACC, some of them joined the BIG and ESPN then sued Fox/NBC for an injunction against televising their games.