I noticed recently that my 5 year old quarter horse gelding has developed a yellowish gunk on his male parts.... Is this normal, or is it some kind of infection?

Is it around the sheath coming out of it.  He may need his sheath cleaned...

 Yes, it is. You can't see it unless he's dropped. I talked to a friend who has a lot of experience with horses. She said he probably just needs his beans cleaned.

It's pretty normal for geldings to get a lot of gunk down there. Here's an article which dispels some myths about sheath cleaning, and gives a short description of how to go about it:

Cleaning Your Horse's Sheath

And, for a giggle, here's something that went around the internet, including the Equisearch forum, a few years ago:

The Sheath Cleaning Song

 Yes, it is normal and natural.  You can clean it off if you want, but I would not bother unless it is causing irritation as it will just be back in a few weeks.  Frequent cleaning really isn't that good for them and no fun for us either!

I just wait for Scout to drop and rub my hand down his weiner to get the loose stuff off. I occasionally remove the gunk which would form the bean. This elicits a strange look from Hubby, so I tell him not to be jealous!