This is a comment of someone who is speaking about Virginia politics without a solid understanding or knowledge of the state, it's makeup, and it's political climate.
Gillespie is a rather lackluster Republican candidate who was challenge, and in a surprise nearly beaten, by a Trumpian style candidate in the Primary. While he ran a campaign that focused a lot on immigration and security, the reality is that he was anything but a "Trump" style Republican candidate. Most diehard Trump supporters in Virginia saw him far closer to the "Establishment" types of McConnell and company than Trump, which made the energy behind his campaign extremely limited.
Meanwhile, there was some motivation and excitement on the part of the Democratic base. You had Obama come out to campaign for one of the first high profile times since he left office. You have the general anti-Republican fervor spurred by Trump being in the White House. The Democrats were running the Lt. Governor of a governor with a very good Favorable to Unfavorable rating.
Add to this that Virginia has been seeing steady growth in the northern region over the past decade, making up close to 10% more of the population in the state now compared to the mid 00's. Beyond Northern Virginia, Richmond and Hampton Roads have experienced the second and third biggest growth in population. Northern Virginia, Richmond, and a large portion of Hampton Roads tends to be a heavier Democratic population. Conversely, the rural areas of the state have been seeing a decrease in population growth over the same period of time. Relative to population, birth rates declined while death due to age increased in these areas. Less people were moving into the rural areas while younger people were more rapidly moving out of them. These rural areas tend to be areas more densely filled with Republican minded people.
What these demographic changes in Virginia has meant is that you are seeing a perpetual movement to Purple that is beginning to verge into truly "blue". It maintains it's purple description for two reasons. The first is because the Democratic strongholds are so isolated but densely packed that it keeps the state house typically Red with a bit of fluctuation. The second is because there is still a substantial and motivated enough Republican contingent throughout the whole of the state that IF properly motivated they can turn out and still cause some surprises or potential surprises, especially if Democratic turnout is depressed (for example, rather bland Ed Gillespie almost pulling off a stunning upset in 2014).
With Gillespie being anything but an exciting and motivating figure to the Republicans in the rural parts of the state, and with the ever increasing Democratic population, combined with a candidate riding the coat tails of a governor generally found to be...at worst..."fine" by most of your moderates and independents and you have a recipe for a Dmeocratic victory.
Had this been Ted Cruz, Marco Rubio, or even John Kasich in the White House, I would expect the result of Northam winning to be no different. I think it's a poor analysis of this race to suggest this is "trumpian backlash" as the primary culprit.