My gelding was gelded young [and for good reason - conformationally he's a bit of a train wreck] and he went REALLY studdy when I got my filly. All of a sudden presence like WOW and sweet-talking everyone with those low nicker-growl sort of noises and licking constantly. He started yelling... a lot... out on rides, got incredibly possessive of his girls to the point of almost running the little pony gelding through a fence, the list goes on.
He's settled down now but for a few days there I had me a stud with no balls. Only thing I didn't catch him doing is actually mounting mares. He is definitely a gelding despite his behaviour those few days.
Some geldings are just studdy. Some studs you would never guess were entire. The later you geld them the more likely they are to be studdy later in life, I worked for a big-name breeder for a little while and was told to be careful of the late-cuts [late-cut meaning cut after covering a few mares] because even though they were "geldingy" 99.99999% of the time you just never really know.
I have seen late-cuts marketed as "for men only", which I don't personally agree with after hearing about my current boss's experience with her stallion [he was dangerous for the man she bought him off, quiet as a lamb for her], but that fact sort of highlights for me that a lot of boys cut after they've sired some foals are more studdy than a gelding cut young.
A CLEAN SLATE FOR THE FUTURE