I'd say go goldsmithing but... honestly it depends because it does mean you'd want to gather if you're gonna craft anything that needs ores or any gatherable mat to save a gil seeing if you buy stuff via market board even if it's cheap the it adds up over time and eventually you will spend a good penny just buying all the time when you can gather some stuff instead and not spend nearly as much. (I'd rather not shove anymore nails into your poor eyes though.) :x
Anyways the reason I'd say go goldsmithing?
They don't always have accessories to make in some levels but they will always be the one class that can do it and accessories = spiritbinding = materias to sell. (Plus most accessories can be worn by any class at higher level and are easier to change to sb than gear would be. Class restrictions changes with gear as certain gear as the player gets higher becomes more restricted than just Disciples of War gears or All class mage gears but doesn't change with most accessories and in crafting they are shared between crafter jobs making them a pretty important piece for any crafter to have.)
tldr; Accessories are pretty flexible to what class can wear them and are often SB'd into materia's which for higher materia's are always being sold strong for least 10k per materia if not higher depending on the materia. (ingots and other stuff aren't bad gil either sometimes but that depends on the server.)
But keep in mind that almost all if not all crafts hit a point where it'll ask them for mats that come from a completely different craft. (Like on weaver I needed ingots to attempt some recipes.)
Edit: Yes of course this won't apply for some people who are willing to gather/craft their own stuff but it doesn't change the fact that people will use accessories for leveling and for sb'ing for materias. :x
Another trick to making gil is to be patient because the biggest way people make gil is selling stuff and making it over time. No one has a real reason to spend any gil on here yet aside from being too lazy to be asked to do the work themselves or being new to the game putting the non-crafters aside so you'll have to just wait for stuff to sell for small chunks and allow the gil to build over time.