I was told that asking about resume gaps is a very biased question and a no-no, especially if it is a single gap, as opposed to a pattern.
Usually the answer to a single resume gap is something personal and none of my business as an interviewer, it's not my place to pry if you were raising children, caring for dying relatives or battling severe depression
That whole line of questioning is a HR nightmare and I don't think they'd get away with it in any large company. You are _not_ supposed to ask open ended questions that could reveal a candidate as a member of a protected group. They tell you that their parents kicked them out because they were gay, they don't get hired, and they file a lawsuit saying that there were forced to reveal their sexual orientation and they were discriminated against because of it.
This scenario is unlikely to be an _easy_ win in court, but presents enough headache and just the risk of the company losing in court would most likely result in a settlement offer.
Usually the answer to a single resume gap is something personal and none of my business as an interviewer, it's not my place to pry if you were raising children, caring for dying relatives or battling severe depression