What you fail to realize is gender in and of itself also holds no truths. It's just a concept made up by human beings. Therefore, how can a truth exist about something that isn't truthful to begin with?
And thank you for the compliment. I am a flower, aren't I?
But see, just because something is made up by humans, doesn't mean it's not real in some sense. Examples: National Borders (Go cross the border into Iran from the mountains on foot with no travel documents and see what happens.) or Money. (If money is not 'real', would please send me your money?)
How can a truth exist about something that is not truthful to begin with? Well, a) You give me no definition of what you mean by truth, but I'll try.
Vacuous truths: More or less consist of an operator on the member(s) of an empty set. E.G. "All of Rangrz's manned missions to Mars have been successful." It's trivially true. Related, are logical conditionals on statements which are absurd/never happened/can't happen. E.G. "The CN Tower is in Kinshasa, If and only if Rangrz is the King of England." Is true on these grounds: The CN Tower is not in Kinshasa; Rangrz is not the King of England. Which is factually true.
Truths can be set by definition/axiomatically."The velocity of propagation of electromagnetic radiation in vacuum is EXACTLY 299 792 458 meters per second." Why? Because the meter is DEFINED as the distance traveled by a photon in free space over the course of 1/299 792 458
th of a second. Ergo, it's analytically true. More analytic truths "This apple is a fruit." Is analytically true because meaning of "apple" already encompasses being a fruit. Or to see it the other way around, the category of fruits, as defined, admits all such objects which meet the criteria of "apple." There exists no object which can be admitted to the set of apples but excluded from the set of fruits.
Other examples of by definition or axiom are Law. "If you possess some MDMA without a license, you are guilty of an offense." and religion "It's a Sin for a Catholic to have pre-martial sex". These are both truth within the framework of that legal system or religious groups doctrine.
Negation of a non-truth is itself truthful. "1+1=2 /\ ~1+1=3".
Another quasi example: Under Newtonian gravitation, an object with a negative rest mass would be attracted to an object with a positive rest mass, the one with the positive mass would be repelled from the negative. The negative one would "chase" the positive one, perpetually. (Perpetual motion) and it wouldn't even violate conservation laws because the motion of the negative rest mass object would have negative kinetic energy, which would balance out the positive's Ke, and the total system energy would be zero. General Relativity says an object with imaginary rest mass would always travel faster then the speed of light.
These are both mathematically true. However, the notion of negative or imaginary rest mass is unphysical and a frank absurdity. Is the statement thus true or is it false?
Godels incompleteness theorem touches on the issue too. Handwaving time (Please, my mathematically educated peers, remember I am handwaving here and don't flame me over the nature of.) More or less, it shows any consistent set of axioms which can provide truths about the natural numbers will always be unable to list all truths about them. Any system that could provide all truths would be internally inconsistent, unable to prove it's own proofs. The lack of internal consistency means that it would also give rise to falsehoods, despite giving truths too.