These aren't the examples you're looking for, there are some similarities between nineteenth-century urban development in American cities and the gentrification of modern cities. If you're really interested, find a copy of David Scobey's Empire City in your local university library, but beware: it is pretty dense!

Big American cities in the mid nineteenth century were fairly unpleasant places: horses and urban farming meant there would be crap everywhere, with little public or private transport streets were incredibly congested, and, without modern heath, fire, and safety codes, disease, fire, and demolition were common. In addition, crime, disorder, and gangs were very common and government often poor. Fire companies - essentially politically-connected toughs - often fought each other for the contract to put out a fire while neighbourhoods burned.

Some propertied urban elites turned to property development as a tool to 'civilise' the urban proletariat they believed responsible for this state of affairs, alongside stronger policing and attempts at disenfranchisement. Projects like Central Park and Eastern Parkway in New York and Fairmount Park in Philly were designed, planned, and built as apolitical, genteel spaces where city-dwellers could come out of the disorderly city and become refined urban citizens. This went hand-in-hand with property speculation and development to the benefit of the propertied elite who could afford to get into the game.

So while this isn't gentrification per se - people aren't renovating brownstones here - I'd say there is a similar underlying issue: whether existing residents or investors/propertyowners control public space, decide what legitimate behaviour is in a city, and ultimately decide who benefits from city government.

I can offer you a few remarks on Rome from the 1st century BC down to Augustus. The dynamic for urban "gentrification" nowadays is significantly different than back then, with the activities largely governed by market interactions at the informational and economic levels. Back in the heydey of Ancient Rome, the gentrification or restoration or monumentalization of certain parts of the city was carried out in a much more formal or planned manner by leading citizens and the very very wealthy. Moreover, although the building or garden might be for private purposes, these would almost certainly be donated to the public on the death of builder or his immediate family. And this is an important difference between modern and ancient gentrification. The ancient model always produced districts, services, markets and other public areas as a public good and almost invariably to encourage the people to have access.

A few examples will help. Following Sulla's proscriptions, the estates of the proscribed wealthy were turned into sources of state income or opened to the public if they were in or near the city itself. That increased state income was then used at the discretion of the dictator to restore temples and build new monuments in the city itself.

Ceasar would widen the Forum, restore the temple of Venus and Mars and build his own temple and house. There is a somewhat cynically humorous story of Ceasar's fellow triumvir Crassus, who, in order to take advantage of the constant threat of fire in Rome due to the construction of multi-storied wooden apartment tenements closely stacked or even leaning against one another, would wait for fire to start and then send his agents into the homes in the line of the fire. He'd buy up all the property for basement bargain prices and just before the fire got there, he'd order his men to knock down all the buildings he had just bought. Thus stopping the fire, and earning him an enormous profit when reselling the freshly cleared new land.

Augustus, needless to say, engaged in almost megalomaniacal building, even if most of his was actually handled and managed by his second, Agrippa. He is said to have bragged that he found Rome in mud and brick and left it in marble. But again it was deliberate, corporate -- state, planned activity ... the building of the acqueducts, the new temples, new market places, new roads, an expanding class of equestes (middle class, very rich) that bought up little neighborhoods and built houses emulating the huge houses of Ceasar, and in the provinces leading powerful men would do the same building fora and baths and temples and colloseums and amphitheaters -- all for very public entertainment, playing, buying and selling, administering justice, etc.