It depends what you mean by gentrification and even then it depends a little on the circumstances, but, basically it's a bad thing. It also tends to lead (and quite inevitably) to the creation of banlieues (there isn't a good word for this in English), which is something that middle class people have a strange tendency to forget.
Why are "slums" more desirable in the inner city, than in an outside ring ala Paris? In all events, it would seem to me to be good social policy ceteris paribus for the lower SES types to have housing near where they work.
Slums on the outskirts are far more invisible, far more easy to ignore, far more isolated as no one who doesn't live there will ever travel through. (Of course, Americans managed to create banlieues in the middle of cities to an extent, but then Americans are ... special. In the bad way.)
Anyways, we usually use the term "gentrification" mostly for the final phase, when a vibrant mix of urban working class and "creative", bobo types like what I grew up in is destroyed as the last of the working class and the poorer of the original gentrifying bobos are being driven out and the place gets dready posh with bars catering exclusively to yuppies replacing the pizzeria I used to go to as a kid and the drinking holes of my wayward youth. Bornheim I fear for you. There's a little too much council housing to drive out
all the working class, admittedly.