This is a good question, and I'm sure I'll miss answering some points, but for the main part-
I think "gentle" comes in two forms, the first being literally gentle. The giantess treats a tiny like something fragile, to be protected. Very cuddly, cute interactions, a tiny guy wrapped up in the larger girl's scarf, or softly kissed, maybe even licked.
Also, a bit of smothering, feet, mouthplay, etc. with consent would count as gentle.
The second form of gentle I would say is used as a way to filter out gore/violence/blatant cruelty. The first two are pretty obvious, but I'd say abusive relationships, psychological torment, etc. would count toward the third category. It's a bit more difficult to properly nail down though, and it can come down to a case-by-case basis.
I may read that story, but I don't have the time right now, so I can't give a definitive answer on that.
If the guy was digested/died inside of her, I'd say no, not gentle regardless of consent. If it was a non-fatal vore, but she tormented and taunted him a lot before swallowing him, that may disqualify it from being gentle in many people's eyes, but at the same time, if he comes out unscathed, and is reassured, then it could swing back to gentle.
Basically, it's hard to create a definitive chart and nail down if something is gentle or not, say "this thing is always X" and "this thing is always Y". The only guaranteed, undeniable time something would be made not gentle is in the case of death or extreme bodily harm brought about willingly, and/or with no remorse.
Really, I could write an essay on it. Overall, it depends on the person, but I lean toward the second definition, most of the time, as long as there's no strong sadism/blood and guts, I'll label it gentle.