I am not in a position to gainsay his conclusions but they seem convincing to me.
From the guardian.co.uk
That may be a task too much, but after coming second last time who is to gainsay him?
From the telegraph.co.uk
This championship was won in a fashion so dominant that no one can gainsay the title.
From the washingtonpost.com
The Pentagon denies it shares that view, but its actions gainsay its words.
From the time.com
Nonetheless, few will gainsay the virtuosity with which Bigelow directed The Hurt Locker.
From the independent.co.uk
No rational person would gainsay that, and it would be bigoted to believe that it does.
From the economist.com
And, you know, who am I to gainsay the personal experiences of a justly famous scientist?
From the scienceblogs.com
No one can deny or gainsay her grit, determination and genuine talent.
From the latimes.com
Almost nobody dares to gainsay him, even when he goes over the top.
From the economist.com
More examples
Challenge: take exception to; "She challenged his claims"
In classical logic, a contradiction consists of a logical incompatibility between two or more propositions. It occurs when the propositions, taken together, yield two conclusions which form the logical, usually opposite inversions of each other. ...
To contradict; to deny, refute; to controvert; to dispute; to forbid
(gainsaying) Opposition, especially in speech; Refusal to accept or believe something; Contradiction; Denial; denying; Rebellious opposition; rebellion
(v) - to speak against, to oppose, to deny
I still confuse "gainsay" and "vouchsafe" because they have the same emotional tone.