practice basically like the above user said..
also PKing in rs depends a lot of the following things
Internet connection
Computer specs
luck
Also, you are getting destroyed by gmaulers because they have 10+ more str levels then you and the gmaul switch/combo is easy and actually really good..
People like myself who have a gmauler and use the dbow/gmaul spec combo do pretty well...you basically get hit with a super powerful dclaw spec (4 hits almost at once)
Also, when I don't have time or don't feel the need for a dbow/gmaul combo then I could just use a normal maul to gmaul combo..
ATM I only have 50 att/70 str..but I still destroy..i get about 4 kills for every one death...That is also with a shitty pc and an internet connection so slow it can't even load rs sometimes or takes about half an hour to load...
This is how I PK on my gmauler (might help you understand why you die)
my invent is basically this, 2 prayer flasks, 1 super str, 1 super att, 1 ranging pot, 1 dbow, 1 p neck, 1 ring of recoil, 1 gmaul, rest of invent is sharks.
My gear is a wildy hood, berserker necklace, regen, dhide, iron platebody, obby cape, 10-14 darrows, ring of recoil, obby maul, climbers...
basically, if i don't get a kill with my spec, I can still easily ko with the obby maul + bneck combo as it hits pretty high (350ish)...
How i normally fight, i will obviously start the fight with potting up...turn on my quick prayers, then when I get the person down to a low health (health low enough to get them to start eating basically) I will switch to my dbow (i never let them see gmaul until I spec with it, I let them think that the dbow is my spec weapon) then if I hit a 150exp+ (it's decent at my level, which is 61cb) hit on the dbow I will switch to gmaul and double spec them as the arrow land...if that doesn't kill them, I will switch to my obby maul and basically just bash them...Use pneck with the other person is specing, when I need some more time to do other things like pot or drink p pot, etc..
The best things IMO to do on your side and not the hardware side of PKing is to practice your combo A LOT..duel arena, even high hp NPCs, etc...