I\'ve not heard the Gigastudio reverb but it is definitely not good. It is optimized for low processor usage.
There are many software reverbs:
Arboretum Hyperverb - Superclean, absolutely no ringings at all but also NOT realistic sounding.
Waves Trueverb - Many settings and good sound, but not Lexicon quality.
Cakewalk FX3 - Very good reverb, a bit limited though.
Sonic Foundry Acoustic Mirror - Very realistic, Very high CPU usage, This is the best software if you don\'t need it to be running in realtime.