Maximum pipe length of a drainage pipe for roof water?

Hello, I have a roof to which a rain gutter is to be attached. The rain gutter still needs to be connected to a drainpipe. The house has two sides - a front and a back. On each side, the roof slopes down and the gable is in the middle of the roof. There is already a rain gutter at the back. I would like to direct the water from the front to the back because I have the possibility to collect the water in a rainwater tank at the back and use it in the garden.

The distance to be covered is about 20 meters, the length of the house from front to back. I would like to guide the water through a drainpipe along the side of the house to the other side. The lower edge of the roof, where the rain gutter will be, is 4m. My question now is, is about 3.5m height enough to guide water through a drainpipe that is to be led at an incline along the wall to the other side over 20 meters, or is that too little, so that, for example, the water might not drain properly?

Write it down better, no one will understand it like this or it's just by chance. The terminology is also somewhat mixed up. I understood that you would like to direct all the rainwater from the roof into a cistern or something similar. The first thought would be simply to run a complete gutter around the roof and use only one downpipe. Whether one downpipe is sufficient depends on the roof area, although I don't know if there are any general calculation formulas or DIN standards for that. What kind of roof shape is it anyway? Gable roof, hip roof? The information that the gable is in the middle of the roof leaves a lot of room for interpretation (or do you mean the ridge?). For a hip roof, what I described theoretically works. With a gable roof, it looks pretty bad if there is a gutter on the gable side, but why not? Usually, one would rather install a downpipe on each side and then lead one of them underground to the tank. You need a slope of 1-2% for drainage pipes. So 1 cm per 1 m. So, I hope I was somewhat helpful, but maybe a few real professionals will chime in here.