i don't think gaastra will rig well on a gun mast. they are constant curve masts leaning towards flex top which unfortunately is not gaastra orientated.
the neil pryde would rig on a gun mast though. i've sailed a couple of the pryde wave sails and they are very comfortable easy handling sails.
edit* you will find though the gun mast is a little stiff for the pryde but only just. should still work.
you could also look at simmer sails, gun sails (not sure if they are available in oz), plus loft and KA, (

) edit* which will suit it better.
the only sail "guaranteed" to work properly is a gun sail though.
my pick, (and yes everyone here knows i'm a KA fanboy), would be the KA kaos. for your age/size/weight it would work very well and they don't cost a mint.
i have a 2008 4.7m kaos that i reckon would get you going as soon as your current sail and a brand new 2011 5.3m kaos. you are more than welcome to rig them up for the day on your kit and try them out. can take the voodoo for a spin at the same time.