Tying Gillespie to Trump in NOVA only might be worthwhile. Even if people say it isn't a factor, it still might be subconsciously. I seriously doubt for many people their candidate choice is actually isolated from their feelings on Trump, although given just how low Trump's approvals are in Virginia, it might be the case that a negative Trump effect will already be built into the race in terms of enthusiasm and other factors whether or not Northam chooses to carry out an anti-Trump campaign.
I don't necessarily disagree, but I think people, especially in NOVA, dislike Trump because of his fundamentally rotten character and corrupt dealings, not necessarily his policies that he's advocated (although those are also extremely bad and unpopular). I think this may drive turnout among angry Democrats who may have skipped off-year elections in the past, but it's harder to convince nonpartisans that Trump is a good enough reason to vote against Gillespie.
Another problem is Virginia never got the Medicaid expansion, so Northam can't even rail against the healthcare bill as a statewide issue as effectively as it would in New Jersey, for example. I think Northam needs to just run the standard VA Dem playbook of going heavy on jobs, schools, and transportation.