Gulfstars were very hit or miss in their design and build quality. In general, the ones from ~1975-1983 were very well designed and quite well built, but those before and after were shoddy. For instance, my surveyor said he would feel comfortable taking my boat to Bermuda, and my rigger says he would feel comfortable taking it around the world. I wouldn't consider it a blue-water cruiser though, and I doubt I'll ever take it further than Bermuda.
I don't know what vintage Triumph was, but it probably falls into the good range. That said, my research into the 50s in particular has turned up many references to wildly varying build quality boat to boat in that model, regardless of year.
I don't think once you cross a certain size threshold, any boat becomes safe for blue water. I think you'll find most people (on this board, at least) would agree that a Hunter of any size is unsuitable for crossing an ocean. You'll also find plenty of people who have made such crossings in Hunters and done just fine, so that doesn't mean the boat can't do it, it's just that it might not have as much margin for safety when things go wrong. I'm hardly an expert on this subject, and I have nothing against Hunters, I was just using that as a common example.
1979 Gulfstar 37 Laissez Faire