In Polikarpov I-153, Were the gulled wings very important?
The I-153 was a development of the I-15, of which 671 were built. However: 


  The upper wing of the I-15 was unpopular with some pilots, as it was felt to restrict visibility, so Polikarpov's design bureau produced a revised version, again powered by the M-25, with a longer span un-gulled upper wing.2 This version, the I-15bis, commenced production in 1937,[7] a total of 2,408 I-15bis' being delivered by the time production finished in 1940.2


So the straight upper wing version was built to improve visibility, and many more units were produced than of the I15.

Structurally and aerodynamically, straight or gulled in this way makes little difference. Structurally, any bend is a weak spot, but the upper wing of biplanes usually had pylons in that position anyway to fix the upper wing to the fuselage. Aerodynamically: the bit missing in between the gull pylons reduces drag a tiny bit - the cross wires have phenominal drag though, and that fat little body also drags a lot of air with it.