Source code in the 8th programming language

: gcd \ a b -- gcd
	dup 0 n:= if drop ;; then
	tuck \ b a b
	n:mod \ b a-mod-b
	recurse ; 
	
: demo \ a b --
	2dup "GCD of " . . " and " . . " = " . gcd . ;

100    5 demo cr
  5  100 demo cr
  7   23 demo cr

bye