vortec spec is .060 as has been said, some situations dictate a change from that, for example with forced induction are better with a closer gap, I ran .045 when I had my whipple on. and variances in the ignition system can dictate gap changes as well...
but yeah, .060 is spec for '96-'00 L30/L31 vortecs.
as far as the E3's and 4+, or whatever...if they work for you and the extra cost is worth it in your eyes, than by all means, go for it, i'm not wanting to start a war over plug preferences... but IMO they fall in the 'gimmick' category and I typically never recommend them...IMO the inability to gap them is a loss of control, as has been stated in this thread, plug gap is something that is dependent on the engine setup, so being incapable of changing that is a really bad thing in my eyes, especially since we're talking about a significant change in gap size (TBI - 0.35/0.45, vortec - 0.060, etc), not to mention other factors like having all that extra stuff shielding the air fuel mixture from the spark to some degree or another (also bad), and the significantly higher cost....I'm not a fan of them. but again, that is just my opinion on them, if they work for you and you like them and they're worth the added cost in your eyes, then go for it.