But a gable roof also requires some form of horizontal.
Gable roof roof bracing.
In the attic go to the gable end and examine the area where the triangular wall meets the rectangular wall below.
Longitudinal bracing members should extend the full length of the roof tightly abut gable and party walls and permit diagonal bracing to pass they may be lap jointed providing the overlap is nailed to a minimum of two trussed rafters.
There are several methods for making these types of connections.
What makes a gable roof what its higher pitch than ordinary roofs.
This higher pitch is going to mean that bracing your gable roof is important.