I really don’t mean to belabor the point, but cooking a 1.5" thick steak on a hot gas grill will get you a seared very rare steak. My experience with the Costco ahi steaks has been that if you want a rare steak then you would cook them for approximately 5-6 minutes per side. If you want well done (no pink) then you are talking about 10 minutes per side. Everyone has a different threshhold regarding degree of doneness, particularly for fish. As already stated, and a position I totally agree with, you do not want to overcook tuna and according to most knowledgeable sources that would mean not going beyond the medium rare stage.
BTW, I have talked with the Costco butchers and, while they cannot sell it as such, they have stated that it could be used for sashimi or sushi. I have personally used it for sashimi now on three different occasions without any ill effects but I would not want to actually recommend that to anyone.
For a steak or a pork chop I go by thickness because to get to medium rare you have to heat the inside a bit. But with an ahi tuna steak I just want a thin seared ring around the outside with the inside pretty much raw like sashimi. 3/4" thick or 1 1/2" thick, I cook it the same, about a minute and a half per side on high heat. Hold it on edge with some tongs to get the sides lightly seared too and it’s good to go for me.
One is a marinade of lime juice, olive oil, and Old Bay - I put it in a plastic bag, add the tuna, make sure it’s coated all over, then squeeze all the air out and let it sit for an hour or so. Broil or grill. YUM!
I sear 1 to 2 minutes per side even for a very thick steak. Then I immediately put it in the freezer to stop the cooking. Once it’s cool, I slice it thin and dip it in soy sauce and wasabi, or ponzu, or sesame chili oil.