Gutters are simply channels that are deeper than the surrounding water. A bit like an under water creek. They can run parallel to the beach or at 90 degrees and any angle between.
They are usually darker because they are deeper.
The most common way to spot a gutter is to watch the waves...Waves break in shallow water. So when you see waves breaking with a gap between them, there is a gutter. Like wise you will often see waves break a fair way out then stop and re-form closer in which indicates a parallel gutter.
I am not sure about fishing for salmon at night. I think it is better at sun rise and sun set, but you can catch them any time of day.
This might help...There are plenty more on Youtube but this one is short and to the point.
itsaboat mate...Life is just a boat and then ya marry one !