One of the neat things about using a gothic arch for this project is that it’s naturally supportive no matter how long the greenhouse is:
Fixing one end of the arc to the ground and the other to the ridge beam forms a natural parabola which equally distributes the load along the beam. The sides hold the ridge beam all along the length so that no other supports are needed. Note that the example above deviated from the true arch and needs cross braces along the midpoints of the arches.