Peter,
Logic is currently up to v5.5 for Mac & PC. In July, it was announced that Apple had bought Emagic and that no further products for PC would be forthcoming. However, Emagic has been supporting their PC userbase and has supplied all new updates since then for both Mac and PC. I'm on Mac and I don't use or know Gigastudio at all, but from numerous posts on the Logic-users group, I gather that folks are running them together, probably with ASIO drivers.
With most VST instruments, their outputs come up directly in Logic and can be processed using Logic's native plugins: EQ, reverbs, delays, chorus, gate, compressor, etc etc etc. Seems lots of people run Giga on a separate computer though, and simply bring the Giga outputs into their soundcard on the Logic computer, via the input objects, so there's no need to commit to files, if I understand correctly.
From where I sit, I'm obviously very attracted by the EXS24 option - it runs from within Logic and you can have up to 64 instances of EXS24 running at once, depending on your CPU of course. You should take a look around the Emagic site for more info:
http://www.emagic.de/english/news/index.html is the main page
http://www.emagic.de/english/products/logic/lap.html is the Logic Platinum start page.
Obviously the folks at VSL believe in Logic, hence their choice to support EXS24.......
Regards - Colin