Gelding is actually pretty uncomplicated and is a great tool, yet it's often one of the most misunderstood parts of the game. The first thing you need to understand is that the majority of male horses on the site are not stud quality. It's expected that at least 75% of your colts will be gelded, potentially more if you want to be especially picky. Note: Before you geld, please take a look at the Stallion Incentive Program (SIP) in the breeding section.
In order to stand at public stud, a stallion must earn 50 racing points (or be a PC-purchased custom). In order to earn any points, a colt has to be racing in at least G3 races and placing 1st-2nd there consistently. If he cannot do that (and cannot pull off at least 4th in G2 or 6th in G1), you are most likely looking at a horse that needs to be gelded. In addition, not all colts who are talented on the track will be talented in the breeding shed. Genetic quality is an important factor to consider too, so you will often find that even high quality stakes racers end up gelded.
It's not just to keep the stud population down. Gelding your horse will have numerous benefits in terms of his racing career. He will gain stats at a faster rate when training, and will accept the saddle twice as quickly as he would have as a colt. Most importantly, his maximum potential can increase and his max morale, consistency, courage, peak, prime, and longevity will improve too. 38 It's not uncommon to see a horse jump up 1-2 racing grades just by being gelded (especially if he was rather low to begin with).
Gelding is also a great tool to better your barn and maximize your profits - however, it's not the right answer for every horse. If a horse has stats that are already pretty high, it will not benefit much at all from gelding. If your horse is already competitive in G1/G2 races, you'll typically be better off keeping him as a stud prospect unless you know for a fact that his genes are less than satisfactory.
How To Geld
Gelding a horse is simple, all you have to do is go to a horse's Manage page and click the Geld button. It's quick and free and will instantly improve your horse. Note: since this increases a horse's maximum stats, not current stats, you will actually have to continue training the horse to see the improvements.
If you come across a horse you're not sure you should geld, good news! Under the Racing tab on the Evaluation page you can find the Gelding Advisor! He will give you his honest opinion on whether or not to geld that horse. However, he does not take certain colors, markings, distance, or surface into account. But, he does take into consideration hidden genes. He tends to be nicer than necessary, so if he doesn't sound extremely optimistic, you'll want to heed his advice to geld.