As part of training you should learn which portions of the POH are mandatory, recommended or advisory.
So if you’re training in a complex you ‘train’ gear up/gear down/gear up/gear down till you’re blue in the face because that is the ‘complex’ part.
If you’re the private owner of the airplane you may choose to leave the gear down in a go-around or when you are doing landings for passenger currency.
The POH allows you to do this.
Examiner question:
Is it required to bring the gear up after take off?
If yes why, if no why?