Not actually played those, which are good ones to pick up? I was kinda put off by how boring, unfunny, terribly designed and written Kings Quest and Space Quest was.
For QfG, I'd recommend the VGA remake of the first one (included in the GOG package) followed by the free remake of the second
; you'll quickly find out if you like the style (RPG/adventure hybrid) or not. Alternatively, go straight for the QfG2 remake; the individual games are mostly self-contained, even though they are parts of one big story. Basically, QfG2 and 4 are the best, followed by 1. QfG3 is a filler entry in the series in more ways than one, and 5 is definitely skippable.
With Gabriel Knight start with the first, which is a traditional point and click (though a lot
less evil than Sierra's games usually are). All three of the games are very good, but you have to overlook some terrible technological decisions in the sequels – GK2 is in pretty bad FMV (admittedly among the best examples of FMV games, but that still isn't saying much) and GK3 sports a very poor 3D engine.