How can you get RuneScape gold

You can woodcut, mine, and fish.
I would say smith and mine is the best way.

Here are some methods;

1. A bronze bar sells for 200 gp, so 28 would get you around 5k each time. Sell it in the grand exchange.
2. Those with a level of 75 in woodcutting with member accounts, can cut magic logs. Selling those for a good 1.3 thousand and about 36 would fit in your inventory.
3. Fishing is very good as well, but nothing competes to the good old art of merchanting...

For non members, you can go to the jail near barbarian village and attack cockroach soldiers who drop law runes/death runes and sometimes even rune scimmys. The only problem is they are level 83, but if you have a group, and food, you can kill them. Also, killing hill giants give you around 330 gp, so does limpy roots.