Any competent welder/fabricator can make a gate like that. Some will also install the posts, too. 6" steel well casing filled with concrete, works well. Have them shield the lock, to make it harder to cut or break off. Do what's necessary to keep people from driving ATVs around the gate.
Friend of mine had hunting ground in Alabama for many years. Was originally a church camp on maybe 100 acres or so, had a coupla cabins and a pavilion. Long lane back thru the woods to get to it, gate out at the entrance to the lane. Darn near every time they drove down there to hunt deer or turkeys, someone had pulled the gate off with a 4x4, or had tried to. So he had heavier, sturdier gates made. Three or four, I think?
Told him the fabricator was probably buying free beer for the kids to party with back there, IF they ripped the gate open, or tore it off, so's he could make another one.
i had one made at a welding shop they came and put it on my pole i used 3in pipe just put it off the road so you can get off the road to unlock it.As far as 4wheelers they will go around gates if they want to they are a problem always
If you already have them cursed multi flora rose bushes on your property, they make great ATV barriers around gates. 25 years ago we had problems with kids on dirt bikes and a few neighbors with horses, accessing our club property via an old roadway on the southern end of the place.
We originally put a dirt pile there to block it, did nothing to keep them out. I eventually uprooted several large MFR bushes with the club's loader tractor and dug them into that dirt pile. Problem solved.