ColorSync on the Mac doesn't actually do anything to change the look of the monitor. What it does is embed a profile of your monitor settings into the file. It also has to have a profile of the printer. It then does a conversion of the color in the document on the fly to adjust between the two color profiles.
That's the theory. In practice, I found that calibrating devices and creating profiles and embedding profiles and the whole crapload of it just didn't really work without quite a bit of work. If it actually worked as well as my description above, then it would be great, but in reality I never got a print out of my Apple Color Laser with ColorSync to match my profiled monitor or the final press output.
Adobe Gamma is just a monitor gamma adjuster. It exists on both Mac and PCs and comes with Photoshop. It's intended as a means to "normalize" your monitor's appearance. I say normalize instead of calibrate, since most people can't tell a calibrated display from a ham sandwich. Unless you use something like a Barco hardware calibrator, it's just a duck shoot. With Adobe Gamma, you are changing not only lightness and darknes, but the actual dynamic range and the curve of where the midpoint of color fits on that scale. It may or may not make your monitor closer to your printer.