Is it permissible to open Christmas gifts inside a game if the game provides them for free?

It is permissible to accept a gift from a disbeliever on the day of their festival, provided that one does not congratulate them or participate in the festival. The Companions accepted gifts from the Majus (Zoroastrians) on their festival. Therefore, if taking Christmas gifts in the game does not involve congratulating them or imitating disbelievers, then there is no harm in it.