Why are there 3 to 6 minutes gaps in my heart rate data on my Apple Watch?

It is a considered thought.

Based on -> Related Article: Monitor your heart rate with Apple Watch

• Even under ideal conditions, Apple Watch may not be able to get a reliable heart rate reading every time for everybody. And for a small percentage of users, various factors may make it impossible to get any heart rate reading at all. But there are things you can do to help Apple Watch get the most consistent and best heart rate readings possible.

I was wondering what algorithms the heart app uses when displaying the hourly heart rate.

I wouldn't know.

Maybe it's your arrhythmia, just to be sure go to the heart specialist.