I’m really trying to do it but I still don’t understand it

gcd(X, Y, Z):- X = Y -> (Z=X->write('TRUE');write(FALSE'))

gcd(X, Y,Z):- X < Y ->(Y1 is Y -X, gcd(X, Y1, Z))

gcd(X, Y, Z):- X > Y-> gcd(Y, X, Z)

Explanation:

The Prolog function gcd() accepts three arguments, calculates the value of the highest common divisor of X and Y arguments and compares the result with the Z variable. If the result is equal to Z, then a true is returned else a false is returned.