Gigs can have multiple instruments, and each instrument may share samples or use unique ones. From what I can tell the red box tells you what percentage of the samples in a gig are loaded into GS3. It's based on the number of samples, not the size of the samples.
One thing you'll notice is that if you already have 100% of a gig loaded, but not all artuculations are loaded, you can load the other articulations very quickly with little or no impact on RAM. If only 50% of a gig is loaded, be prepared to use more RAM for that next articulation.