1. Mentioning Gab as a possible site to sign up for is "pretty blatantly out of line" and a violation of the Stack Exchange Code of Conduct, and
2. If they discovered that a job candidate had a Gab account, they would throw out the application based upon that fact alone.
So it's not just internet communities; we've got academics openly bragging that even engaging with a community they politically disapprove of, regardless of your individual views, will lead to them barring you from employment in academia.
Even assuming that's true (and I don't know or care if it is), it's unclear to me why it should reflect on the community. If tomorrow somebody were to leak a tape of Paul Graham or Joel Spolsky ranting about their hatred of some race, it wouldn't somehow reflect poorly on the character of anyone with a Hacker News or Stack Overflow account.
First and foremost political opinions have no place in most professional settings and no influence on someones work. If I recall correctly it's even illegal to judge someone based on their political affiliations in many countries.
Further, someone could have an account there to comment against the radical opinions or because he has friends with those opinions, which brought him to the network. And surely some more reasons why someone might have an account without sharing the extremist views of the outliers there.