this should clear your all doubts

var gdt = new GlideDateTime();
gs.info(gdt);                    //UTC
gs.info(gdt.getDisplayValue());  //local instance time
gt = gdt.getTime();
gs.info(gt.getHourLocalTime());   // The hours using the local time zone. The number of hours is based on a 12 hour clock. Noon and midnight are represented by 0, not 12.
gs.info(gt.getHourOfDayLocalTime()); //The hours using the local time zone. The number of hours is based on a 24 hour clock.
gs.info(gt.getHourOfDayUTC());      //The hours using the UTC time zone. The number of hours is based