The General Classifications (plural) are outlined in Article 25 of the TdF Rules; they include:
• Individual time classification (Yellow Jersey);
• Individual points classification (Green Jersey);
• Best climber individual classification (Polka Dot Jersey;
• Young riders classification (White Jersey);
• Team time classification;
• Most aggressive rider classification (Red Number);
• Centenary prize rankings;
• Enlarged Europe prize.
However, as a practical matter "GC" has become synonymous with the pursuit of the overall lead of the tour, aka. the Individual Time Classification and the Yellow Jersey. The "Individual Time" GC is therefore the list of all riders who remain in the tour (150 or so today), sorted by their elapsed time with the lowest time being 1st. If you look at the "Individual Points" or "Individual Best Climber" GCs lists you'll also see all 150 or so riders listed by highest points first. If a rider doesn't make it to Paris on Stage 20 of the tour, they are no longer in the General Classifications or elibible to receive the prizes associated with each of the general classifications.