Darren,
Perth is definately one of your best options to target Samson fish at this time of year. The last month or so fishing for Sambo's has been red hot, but it's hard to know how long it will last for. I went out jigging a fortnight ago and between two of us accounted for 18 fish in about 3 hours averaging 25 -30 kgs. with one standout being around 40kgs.
In terms of GT's you would really want to begin at Exmouth, that is where they start becoming more prevalent, and in terms of charters who target them I would not know who to contact as it is not something that is all that widely done here in WA.
Steve.