Some of the Digidestined could be cautious. They've faced many challenges in the Digital World and might be worried that Gallantmon could be a new threat. However, their digimon might sense Gallantmon's noble nature and try to communicate that to their human partners.
They might be in awe. Gallantmon is a very powerful and majestic digimon. They would probably be a bit intimidated at first, just looking at his large and imposing form.
One idea could be that the Digidestined are in a dire situation in the Digital World and Gallantmon appears as an unexpected savior. He then decides to guide them on a quest to find a powerful artifact that can restore balance to the Digital World.
In this fanfiction, the characters could interact in a very competitive way at first. Each group might think they are better at handling Digital World problems. So they could have little contests or challenges to prove who is more capable. However, as they face a common threat, they would realize that cooperation is key. They would start sharing their skills and knowledge, like the Digidestined teaching about Digimon training and the Frontier kids showing how to connect with the Digital World on a spiritual level.
Some of the gaang members might be very excited. Aang, being the Avatar, would probably be curious to learn from Ursa. He could ask her about the Fire Nation's history and how it relates to the balance of the world.
Luke might be in awe. He's always been seeking his family and a sense of belonging, so meeting his parents could make him feel complete. He'd probably listen intently to whatever they have to say.
Some of the Avengers, like Thor, might have a more complex reaction. Since he is Loki's brother, he may feel a sense of responsibility towards the children. Others like Captain America would be cautious but fair, trying to assess if they are a threat. Iron Man, on the other hand, would probably be sarcastic and make some snarky remarks while sizing them up.
They might be cautious. After all, they don't know her and she's from Percy's world which could be full of unknowns for them.
They would probably be very concerned. Jungkook might be the first to approach her and offer his help. They'd do whatever they could to make her feel better, like giving her a warm hug.
They would probably be shocked. Demigods have extraordinary abilities, like being able to control water or run super fast. Percy's mortal friends, who are used to a normal world, would be taken aback by such powers.
Thor might be conflicted. He'd see his nephew/niece but also remember Loki's treachery. Iron Man would probably be sarcastic at first, making remarks about the family genes. Captain America would be more cautious and diplomatic, wanting to give her a chance but also being on guard. Black Widow would try to read her body language and motives right away.
His family would probably be overjoyed. After all, they might have thought they'd lost him forever. There would be hugs and tears, and a flood of questions about his life and what he's been through.