I Made a mistake on the location submitting a POI, which ended up being accepted but it is on an unaccessible position, which means that I cannot get near it to report it. it also became a gym, so that it is empty (a good indicator to show its unaccessibility?). How can I have it removed? So that I can submit a new nomination with the right location?
You can use help chat at Niantic Wayfarer and give them the correct coordinates - with evidence. A text box will pop up and a bot will take information, but a human will reply. Be aware that, depending on the inclusion rules at the actual real world location, the gym may no longer appear in the game, but reporting the correction is the right thing to do.