Knew I should have finished getting my certification.
While I used to work in prefab shop many years ago, I do my own work but don't ask me if I'm a welder by trade.
Gussets are triangluar sections welded into a corner for more strength.
Let me see if I have this right. You want to take a section of 2x2 box iron and weld (2) 3/4 tube sections to the ends? If you use box iron with a wall thickness of 1/8 inch or more you can probably hang the whole boat on it so you won't need gussets. Punch (drill) 3/4 holes through one side of the the box iron, only 1 side, square it up then full circle weld that outside joint, tack the inside joint on the 3 sides facing to you skip the one in the back, your done.
As for pipe strength and thickness use only schedule 40, 20 is weak and 10 paper thin. Look at pvc to get an idea of what I mean.
If haven't welded before don't bother it's no where near as easy as some make it look. And for the love of GOD please use a full face welding helmet or you will super sunburn your flesh off. Not to mention you might end up permanantly blind.
"We're going to take things away from you on behalf of the common good"
Sen. Hillary Clinton - Speech at Democratic Fundraiser, June 2004