The G-word always invites controversy and confusion, but from the authors own work:
To assess gentrification, longitudinal analyses were performed to examine median household income, percentage with a college education, median housing value, median gross rent and employment level.
Hutchings H, Zhang Q, Grady S, Mabe L, Okereke IC. Gentrification and Air Quality in a Large Urban County in the United States. Int J Environ Res Public Health. 2023 Mar 8;20(6):4762. doi: 10.3390/ijerph20064762.
There are issues with assessing geospatial phenomena using ZIP code level data, but these findings give pause and hopefully encourage a greater deep dive.
DETROITography 
What does “ineligible to gentrify” mean? I always assumed gentrification was a de facto phenomenon and that therefore there is no authority in charge of setting eligibility criteria. The gray districts seem to be the “outer city” areas where the housing stock isn’t old enough to be cool enough for the yuppies; the “This Old House” crowd. I worry about the large tracts of ranch houses and other postwar ticky tack, both inside and outside the city limits. Is that to become the new ghetto?