This really should be in ELI5, but consider:

Gentrification is when rich people break up a poor community. For example: if you were a poor Mexican kid living in San Francisco's Mission District in the 1990's, you'd be poor & Mexican (which sucks); BUT, you'd know your neighbors; your parents would know your neighbors; you'd go to the same church your grandparents went to; you'd go to school with your cousins.

When you needed an afterschool job, your folks would talk to the owner of the supermarket where your family had shopped for 40 years about getting you a job. When you're bored on a Friday night, you and your cousins go bowling with your uncles. If you got into trouble at school, your teacher would tell your uncle (who is in his bowling league) who would tell your parents, who would tell your priest, and all of a sudden everyone's on your case to straighten up.

Communities build networks; networks build institutions; institutions build support networks.

Now suppose the rent goes up because the Google Bus comes to town, and your parents move out to Milpitas. They're gone 18 hours a day (because of the commute) and no one knows anyone in the neighborhood. You get in trouble at school--no one finds out. You start running with a bad crowd--your cousins aren't there to steer you straight. Your priest tries to steer you straight, but you can just lie, because he doesn't know a damn thing about your life. When you need a job, there's no one to turn to.

The loss of the community brings real damages to people's quality of life. That's why gentrification is a big deal.

I have no idea what the second part of your question even means. I'm an American, and it's not a homogeneous place; never has been, never will be. Until 1846, San Francisco's Mission District was part of Mexico.