It all depends on the metal you are using - soldering would probably be out of the questeion but you have the options of welding and brazing.
Welding is when two pieces of the same metal are heated to the point where they sort of "melt" together. Of course they do not quite melt but they are heated to the temperature where they are close to melting. So that is probably what you would do with gate or cheap bicycle frame.
A fine quality bicycle frame needs to be stronger than that and is brazed - pretty much the same technique as welding - tubes heated with a torch, but a different metal is used to join them together, but you don't need to heat the tubes to point where they are glowing - meaning the structure of the metal stays intact and they are stronger than if you would weld them. The temperature is higher than with soldering. Often silver is used to braze steel parts together. You'll need more than 100 kg per square mm to tear that apart. You'll never reach that quality with CO2 welding.
This is only from a translators point of view - for details ask somebody who has actually done it.
Not all metals can be welded. Iron, steel and aluminum can.
If it is a very decorative gate with brass parts, they are definitely not welded.
What is funny, is that many languages, German included, only have one word to translate the two different techniques, soldering and brazing, so when somebody writes that word in a text that is being translated into English one often has to pick up the phone and ask the author what is actually being done. Or you only have a 50/50 chance of picking the right term.