Gussets are angled support parts, when you have a vertical member connected to a horizontal member, the gusset is a fraction of the length of the first two, and installed inside so as to create a 45-45-90 triangle. The purpose of the gusset is to prevent racking; which makes a structure change from square to a parrallelagram.
Your pallet base will last for years, especially if you set about six patio blocks under the runners or half sink some bricks into the ground under the runners.
The uprights will bow very little once you have some gussets between them and the roof boards, and those would give you much greater benefit if they were rotated 90*, but since you don't have snow or ice to worry about, they should be just fine as they are.
Typical pallets are built with the least material to achieve the greatest strength, general principals are followed.