"It's all painful, I'm a human being but I bury this deep inside me," he said, adding that his task is to "help people as much as possible".
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Conditions are grim for civilians left in Lysychansk, who have no mobile phone connection, running water or power. They cook on campfires and shelter in cellars.
"About 10 percent" have stayed in the city, Gaiday said.
"We try to talk to people and persuade them to leave. Some point-blank refuse."
There is a "small percentage" who are waiting for Moscow to build a "Russian world" in the region, he said.
Gaiday has a prominent presence on social media including Telegram and Facebook, giving regular updates on the war.
"You need to talk," he said, saying this could counteract the powerful influence of Russia's state propaganda machine.
He said he also wanted people in the conflict zone to "understand that I haven't abandoned them, that I'm on the spot and with them."
Referring to a potential war crimes tribunal, he also suggested his posts "could even be a small element when we try (Russian President Vladimir) Putin at the Hague".
