A mitre joint is the standard corner detail in gate frames where both members are the same RHS or SHS profile. Both members get cut at 45 degrees on a cold saw or band-saw mitre bench, butted together, and welded full-perimeter to form a square corner with no end-grain visible.
Mitre joints are stronger and cleaner than butt joints (where one member runs through and the other lands on its face) because the weld runs around all four sides of the tube section rather than just three. They also look cleaner in finished, painted gates. No visible weld sits on a hard edge.
CAD60 drawing packs show mitre cut angles on the side elevation and in the cutting list, ready to feed straight into the workshop saw.