The leader's birthday party is for everyone to attend. It seems that such a leader is very approachable. Since the leader has invited you, you must not refuse casually. Whether from the perspective of politeness or other aspects, it is necessary to bring a small gift. Here are some suggestions for you:
1. Ask your colleagues what kind of gifts they bring. It is best not to be too different from the gifts brought by everyone. Just keep the same level of gifts as everyone else. Otherwise, it will be too abrupt and will not look good on others.
2. It seems that your leader is kind. I suggest that you can give some moderate gifts, such as books, or some simple ornaments. Crystal is good for female leaders, and rock ornaments are good for male leaders.
3. In fact, sometimes the leader says that inviting everyone to dinner on his birthday is just an excuse. He just wants to get closer to everyone. You can bring a bottle of wine, which is also good.