Updated and corrected answer:
Gold spawns between bedrock and height 32. Between y=2 and y=29 you will find the most gold.
There are two methods I will recommend to quickly and efficiently obtain gold.
METHOD 1 - Branch Mining
Dig down to level 11 or 12 so that you're at or above the lava lakes, so that you don't have it pouring in on you all the time. Make a corridor in either the x or the z direction. Every 4th block along the corridor, make a tunnel leading away from the corridor. This is the best way to find nearly all the ores on that level while breaking the minimum number of blocks.
The reason I recommend going to this level is that you will also be finding diamonds, as they spawn below level 16.
If you wanted to more thoroughly mine the area, do your first mineshafts at level 11, then go up to level 14 and do mineshafts in between the ones below, so that the tunnels are arranged as follows, where x = tunnel and s == stone:
This will allow you to find virtually all the gold (and diamonds) in those levels.
METHOD 2 - Speed Mining
This is a more end-game method, as it requires the use of a Beacon and lots of diamond/xp levels.
Dig down to bedrock (level 5) in the center of the area you want to mine out. Place a full beacon at the bedrock level, and set it to Haste II. Get yourself an Efficiency
V pick, and a Silk Touch pick. Then proceed to mine out ALL the material in the beacon's coverage area (50 blocks away from the beacon block), mining stone with the Efficiency pick and non-smelt ores with the Silk Touch pick. Start by standing on level 14 and mining blocks 15 and 16, then work down from there. You will find TONS of gold and diamonds this way. If you convert lava to obsidian and mine it, you will end up with around a double chest of obsidian as well. Throw the gold and iron in furnaces, and use a Fortune III pick to break the ore blocks later, so they don't fill up your inventory so fast when you're mining.
Another way to do it, now that repairing tools costs so little in 1.8, is to just use an Efficiency V Silk Touch pick, so that you don't have to carry two picks around with you and switch out all the time.
If you have two beacons, it's even faster if you set one to Haste II and the other to Speed II. You can sprint mine and get a lot done quickly.
I have done this in two different areas, and I have around a half stack of diamond blocks, even though I've used tons of diamonds to make tools and armor.
Finally, after 1.10, an additional 20 veins of gold per chunk are generated in Mesa biomes between layers 32 and 79 inclusive. This means on average an additional 120 gold ore per chunk on top of the normal 8 ores. For comparison, there are about 77 iron ore per chunk.