Thanks for your answers.

Remember toe nailing into Douglas fir and splitting a good number of
2x4's...

Probably will make the gunnels 7/8x1 1/2.

I thought to follow the instructions on wolfgangbrinck.com: dowel in the
deck beams and use flat ribs placed 6 inches apart. The mortises for the
ribs are 1/4x1 and 1/2 inch deep. I guess the ribs are 1/4x1 also.

When laying out spacing of the deck beams and ribs on the gunnels would it
be alright to move the beams foreword or back a bit away from the rib mortises?

Kirill