As long as you have the 2GM20F
there are no zinc's in the engine. It seems this question has come up many times before with the same results of some saying that the Yanmar GM series fresh water cooled engine doesn't have zinc's, and others suggesting otherwise. Check out this thread at the bottom of page 1 and top of page 2:
(There is no) Zinc pencil on Yanmar 3GM30F
There are other places on the internet that say the same thing, but here are the results:
Yanmar uses cupronickel heat exchangers in all the fresh-water cooled GM series and do NOT need zinc's. Then again just try to buy the zinc for a 2GM20F, you won't find it.