GoldCoin difficulty is adjusted using a block solving median time every 60 blocks. The median time is the time that would be in the very center of a sorted list of ascending block times for those 60 blocks. The median time is not the same as the average time.
For instance lets say that our difficulty algorithm adjusted every 3 blocks (for simplicity). Let's say those numbers were 145, 50 and 100 seconds for the time to solve each block. The average time would be about 98 seconds, while the median time is 100 (because when sorted the block times are 50, 100 and 145 and the middle number is 100). Then the difficulty would be adjusted by a ratio of 120/100 to make the network difficulty higher, because the difficulty was too low to get an median time of 120 seconds (2 min block time). Other coins, such as bitcoin and litecoin use an average block time for 2016 blocks for their difficulty calculations.
Many coins are now using the Kimoto Gravity well to adjust difficulty. It is an interesting algorithm. It does a difficulty adjustment every block. It's calculation it looks back at a certain number of previous blocks (which differ based a second set of calculations - the gravity well). It may look back at 144 to 4032 blocks on Megacoin. Other coins look at fewer blocks. The coins are trying to prevent multipools from strip mining and dumping the coins.
Goldcoin doesn't need to worry about Multipools because of its defense system that prevent more than 6 blocks from being solved in 10 minutes. The multipools would usually prey upon coins that adjust their difficulty after long periods of time (or many blocks) and the multipool can come in with its 1 GH/s and mine hours of blocks (normally) in minutes. The kimoto gravity well may actually prevent this. That doesn't mean the multipool would stop mining those coins, just that they won't get as much profit out of mining them.
