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.
Well, it's good social policy for
everybody to have housing near where they work, for myriad reasons. To the extent that it forces lower SES types to have longer commutes, that's really the one thing about gentrification that I would consider to be a likely negative.