Offensive Strategy:
Note that dealing a small portion of damage will not make the Gol Dragon lower its head, like the Dragon. After all, this is another enemy we're dealing with, so what did you expect?

Hunters: Grab your strongest Saber, Dagger, Slicer, and Handgun. While the Gol Dragon is walking around, stay on its sides, a little bit behind its feet. It's best not to be close when it'll stomp to raise the ground, but other times are fine. Against the clones, wait for an opening and use a ranged attack.

Rangers: Use your normal array of weapons: Handgun, Rifle, and Mechgun. Use a Rifle to stay at a distance, and when you can get close enough, use a powerful Mechgun.
Forces: Gol Dragon hates Barta, but like any ol' boss, you can use Gizonde to hit multiple areas. Gizonde can make the fight a breeze. Use a normal Zonde as usual when it's in the air, and I'm sure Gizonde can hit the clones along with the real Gol Dragon. Jellen and Zalure do not affect the Gol Dragon.

Hard, Very Hard Strategy:
The addition of a second clone in Very Hard and Ultimate requires your full patience, but the fight is not impossible. Just deal as much damage as possible.

Ultimate Strategy:
The Gol Dragon is weak to Foie.

Gol Dragon is fought quite closely to the Dragon from the Forest, save for some attacks. Many of the offensive strategies apply, though the defensive strategies are quite different.

The first thing is Gol Dragon's elemental breath. Fire isn't a big threat, but thunder comes next, and ice is the one to worry about. Sol Atomizers is required to eliminate the paralysis while high EIC, a Cure/Freeze unit, or quick fingers is needed to eliminate the Freeze status. Ice resistance can lower the chances of being frozen in place, Cure/Freeze can fully prevent Freeze, or quickly wiggling the analog stick back and forth can get rid of it quickly.

First things first: the instant the fight starts, immediately retreat. Gol Dragon will always use its breath to surprise you. Try to make it so that you can keep Gol in sight while running away. After it stops, you can run towards it, but beware of it phasing out. Next, it'll fly up momentarily and stomp the floor (careful of the rising photons!) and attempt to afflict an element with its breath again.

After that, it'll do the most dangerous attack yet: it'll fly up to the center of the arena and start spinning itself, flying off either one element or two different ones! At this point, the safest spot is the very edge of the arena, so when you see it fly up, run for the edge.

Taking away half of the Gol Dragon's HP will prompt it to create a doppelg?er of itself in Normal and Hard, while creating two of them in Very Hard and Ultimate. It's literally going to difficult to avoid any attacks, and you have to keep an eye on the patterns that the Gols are doing. If you can detect a safe point to run in and attack, do so.