My first time in Gold I didn't even know it was a Gold match until I'd died for the second time on wave 1 :D Luckily it was GWG, so I found everyone else ad we finished the match properly. Now I'm addicted to GWG :3
If you take a pair of d claws to troll invasion, you can abuse the potion there and just train strength through claw specs, last I heard it still works.
Hellhounds hit rather hard for their level. I'd suggest barrows or bandos armor with a bunyip and food in invo, since they don't have any drops aside from bones, charms, and clues.
Magic is trained more easily because magic spells have a base experience, along with experience for damage, whereas range and melee only get trained by actually dealing damage.
What's unclear about it? As for best xp, I find camping the best wraith I could siphon while siphoning any nodes around it worked best. All nodes are better xp than just siphoning wraiths, with the exception of air nodes, which just come under slightly.
They contribute towards your Runespan points, which you can spend with Finix to buy wicked robes, massive pouches (which aren't worth it imo) and cosmetic esteem. This is all mentioned in our runespan guide btw :)