It's when someone does or becomes something they're usually not, like what you eventually see is different from your first impressions.
For example, a tsundere is a great example of gap moe. They tend to appear cold and fussy, at first, but deep inside they're actually warm and fuzzy. Eventually, you'll see that side of them and you'll think how different they were from before.
Another example would be appearance and hobbies. For example, a tough-looking man actually likes teddy bears and cute stuff.
So, yeah! It's things about them that you didn't actually expect, which you find very likeable.
please recommend gap moe bl where the character is lowkey an asshole but looks like an angel, preferably manga, or tsundere bl manga ples