I think the difference with SI is the swimsuit issue is a defined issue, promoted in advance, for a magazine that covers all sports. GD is a golf specific and targeted at only golfers. GD may have gotten a bit of interest boost with this, but in the long run, I fear it will hurt their sales.
Actually, I think that is an unfair comparison towards GD because the SI swimsuit issue has been around for 30 or 40 years (whoops, just googled ... actually 50 years old!!!) and has BECOME a defined issue over time, promoted in advance, and for a magazine that covers all sports.
But I even remember in the 80's (when the issue was apparently already 20 years old) it was still considered controversial to some because, come on, suntanning on the beach is no more related to sports than Paulina Gretzky is related to golf. Over time it developed into what it is today, which is an entire issue** devoted 100% to beautiful women in (and out of) bikinis.
**It used to be just one "article" out of the entire magazine that still had sports stuff in it.
If GD continued with this "fitness" thing, tried to make it a tradition with a hot girl on the cover every year, who knows, maybe it could turn into something positive. (Next year, will it be Paris Hilton, whose affiliation with golf is that her family owns some hotels that are part of golf resorts around the world, or will it be Kim Kardashian, whose dad used to play golf with OJ a lot? )
But if you're going to compare this to the SI swimsuit issue, I think you'd have to compare it to the issues from the 60's, and it's a safe bet that the reception was pretty similar for a lot of people.