you must first know something about the nature of the person giving advice. You see, I would weld in diamond plate myself... the same in a 3/4 ton working truck or an oil field truck... 10 gauge is about 1/8th inch (over on a boat building forum we just had a lengthy discussion on Imperial vs metric). 12 gauge is considered "heavy", 14 is acceptable for horse trailer repair (the front curved part just above the floor rusts out). I have heard modern auto body work trashed as being 22 or 26 gauge (probably 26 or 28 by now) but I do not actually know that to be true, besides those panels are all curved and bent edges. If you can make grooves / ridges in it for strength then think 16 or 18. How wide of a span without support is also a factor obviously and a factor in "drumming". If span seems to be "big", add some cross bracing and stay thin. You can use a hand drill to make holes for "spot welds" but I took a bench mount drill press and cut down the post (and turned the drill head around) so I could drill below the intended drill travel. Yes, these "toy" wire spitter welders are sufficient for you job if doing Thin (but they are not good for much else except a tack to hold pieces until you do a real weld. The set up using solid wire and CO2 bottle (small / cheap) is so much cleaner than flux core that you REALLY should go that route. I have a friend in the ornamental iron / gates and gate art bidness and he uses TIG so he does not have cleanup. You do not need to go that far. Skip around (literally) all over the place with short welds and then 1/2 way between those spots after they cool and then 1/2 way between again and again . People will be impressed with the job you did. I assumed that if you are asking about gauge then you are not experienced in these matters... I apologize if you were just looking to see what others think. .
Beg borrow or steal a Plasma cutter. Use a straight edge or other guide and it will look like NASA quality work. Maybe more than you were looking for? Tio in Paradise on the Rio