"Runescape" does not have any cheat codes for gaining gold, but one way to make gold easily is by flipping goods. For example, buy some feathers from a merchant for 1,000 gold, and then sell them at the Grand Exchange for 17,000 gold.
A few other ways to make gold are selling iron ore, selling Hill Giant bones, and picking bananas. However, the amount of gold earned in this way depends on the current market prices.
The game uses server-side character data, which prevents players from cheating. If a user cheats or uses exploits, it will likely result in a permanent account ban.