In the 1890s, Mrs. Charles Knox made her own gelatin by boiling left-over slaughter-house waste of cow's hooves, bones, tendons and skin. She and her salesman husband devised a method of drying sheets of gelatin and grinding them to a powder. They claimed that gelatin would make fingernails stronger, presumably because cow hooves are strong. You can still buy Knox's gelatin, and some people still believe the old advertising claims, even though there is no evidence that gelatin has any effect on nails.
Gelatin does not contain any special nutrients. It contains protein, but lack of protein is not the cause of brittle and cracked nails. Even if protein deficiency caused brittle nails, gelatin would be a poor choice, because it is very low in two of the protein building blocks, tryptophan and lysine. You can meet your needs for protein by eating any food source of protein, such as beans, meat, fish or chicken.
If you have deformed nails, check with a dermatologist to see if you have a fungus infection that can be treated. If you have nails that just peel, crack and break, you probably have a genetic defect that causes your nails to lose moisture. The most effective treatment is to use nail polish to slow the loss of moisture, and to keep the nails very short.
I'd heard that eating lots of gelatin would make your nails thicker, so after I slammed my finger in a door and my nail fell off, I started eating LOTS of gelatin in the hopes that it would make my nail grow back faster. I didn't notice a significant difference until I'd stopped eating the gelatin. Shortly after I cut back on my gelatin intake, I realized that there was a visible ridge where my nails got thinner. There hadn't been a visible line when I started eating the gelatin, but there was a very clear line from stopping eating the gelatin. Apparently eating lots of gelatin does thicken your nails.