smooth it totally with no seams showing or just so you can camo it?,,if you don't weld the entire edge of the `patch' panel we'll call it it will eventually show back up as a crack in the bondo due to flexing ANY `skin' like a quarter panel for instance should always be welded all the way across any outer area rather than tacked,car bodies vibrate and they flex and they expand and contract with temp changes `bondo' is just fiberglass resin and talc it won't `hold down' two pieces of metal,wellllllll i guess i could

for awhile!
i've wrapped tailgates with `tin' and smoothed up, chopped up, frenched in, and molded in parts on cars for 30 years and rebuilt wrecks as well as done insurance repair work=i'm not being a smart-a-- just trying to help
should be easy if you just want to make it look smooth from a distance(with seams),drill holes in the patchover panel and just spotweld it on the old gate/grind the welds and camo it!!!!,,,otherwise(here we go

)=make sure the gate is either on the truck(best) or setting on a level work bench(gamble)tack it ONLY in one spot then move to the other side of the gate,never let the heat build up! and buy heavier guage steel than you think is `good' (18guage?)eventually you'll end up with one inch sections you havent welded by moving around and letting each section cool COMPLETLY,take your time!,,,,,,,,,,,,,i'll give you an example=put some heat on an old car part=don't matter what car or what part,you can use anything even a propane bottle=watch the metal expand

you'll see that welding on car bodies takes time or when it all cools it gets ugly
they make em out of `tin' so they can stamp the panels out,body lines create strength and hold the skin shape on the outside,,,,,,,,,,totally sorry for the longwinded post here but maybe somebody can get somthing out of my 30 year investment=anything is possible! look at the old ledsleads from the 40's50's and 60's

,it'll prolly weigh more!