They're very different - two reasons, not exhaustive. First, immigration doesn't force natives to leave in the way gentrification opponents claim gentrification does. It's not the moving in people dislike about gentrification, but being pushed out.

Second, nationalism is legitimate in a way localism isn't. The borders of a part of town are frivolous compared to the borders of nations, both in practical respects and in terms of normal human bonds. Ironically, the opponent of gentrification is the person with the least solidarity - the fortunes of his neighborhood and neighbors improve but all he does is moan "Me, me, me!" Is it any wonder their neighbors are glad to trade the pro-squalor activists in for people who are pro-improvement?