I've posted about this in thread before but briefly: what has been gained, as others have rightly said, is equality. Before this, gay couples were classified differently by law. They could not say they were getting married, because they were actually getting 'civilly partnered'. People could say 'but you're not married are you, because you're gay and gay people can't get married'. They couldn't call each other husband, or wife. They weren't recognised as married partners in other countries that have already passed equal marriage.
They were told they were being the same, but different. That sentence right there was the basis of segregation in America. I know gay couples aren't being made to sit at the back of the bus, but they are subjected to abuse and discrimination, sometimes even from our national newspapers in recent years.
Changing the law to support equal marriage starts to normalise it. Now, gay couples can get married legally like a straight couple. Now, children are not brought up in a society where they see gay couples being treated as different by our own government. Now, if a transgender person got gender reassignment treatment with the full support of their married partner, they would not be forced to divorce.
This is a huge huge deal, and the fact that you don't see that shows that you are very lucky to have never been or never known anyone to have been affected by these laws.
Remember it is only in the last twenty years or so that gay people have been given the same rights in workplaces with anti discrimination laws and not much longer since gay people were allowed to have sex at the same age as straight people.
Hopefully, with equal marriage, we are on the way to a future where a gay person doesn't feel the need to come out almost every day of their life and gay people aren't verbally abused in the street for holding hands with their partner. How can gay people have equality in society when they don't have equality in the law?