What's the name of the gift to congratulate the opening?

This is called the opening red envelope.

Red envelopes, also known as red envelopes and gifts, are small gifts made of red envelopes. They are often used in various celebrations under the background of China traditional customs and Chinese character cultural circle, including Chinese New Year, weddings, birthdays and other occasions, and their colors represent the meaning of "celebration" in China culture. Since the Ming and Qing Dynasties, the practice of wrapping copper coins (begging for lucky money) has gradually evolved into a modern gift-giving form of putting money in red envelopes.