Elevated gutters, is becoming more the “norm” when building new greenhouse structures. It is an important decision to make and comes with extra expense however, this expense can be offset as a traditional internal drainage system is not required when growing from the ground.
There are other floor gutter options available that require a perfectly level floor. If the floor isn’t level you will end up with potential troughs where the water sits. If you decide to slope your floor, to compensate, remember it will potentially create temperature variation.
Gutters offer you a large saving in labour. It is easier to put substrate on and to remove from elevated gutters than the ground which is friendly on the back. It is far easier and efficient when planting. For a period after planting (time varies depending on what vegetable you are growing) it is very easy to perform the initial jobs like clipping/tying up the plants and carrying out important plant maintenance.