gcd(X, 0, X) :- !.
gcd(0, Y, Y) :- !.
gcd(X, Y, Z) :- X > Y, !, X1 is X mod Y, gcd(X1, Y, Z).
gcd(X, Y, Z) :- X <= Y, X1 is Y mod X, gcd(X1, X, Z).

gcd(8,2,2). --> true
gcd(8,3,2). --> false
gcd(3,8,2). --> flase

gcd(2,8,2). --> false

gcd(8,X,2).

ERROR: Arguments are not sufficiently instantiated
ERROR: In:
ERROR:    [9] 8>_5726
ERROR:    [8] gcd(8,_5752,2) at /home/zula/prologme.pl:5
ERROR:    [7] <user>

?- [gcd]

pl:6:18: Syntax error: Operator expected