So I just had to reply to this thread.
Heel-toe is left foot breaking while clutching to maintain RPM's.
Since GT5 doesn't use a clutch it's pointless. If you have a GT27 (I do) or other pedal with clutch - it acts like a gear. You can go into that gear and out by changing gears but you can not slip the clutch. Which would be great if you could.
Less I am the only person who's clutch doesn't work ...
I don't quite get what you mean with that sentence in bold. You have to brake with your right foot to heel and toe.
You don't need clutch slip to heel and toe. It's all about blipping the throttle to match the RPMs for the lower gear and thus eliminate the awkward engine braking which happens when you release the clutch and reduce the forces going through the car while braking which makes it more stable and braking more effective.
I don't think it's absolutely neccessary to do it in GT5 especially if you use ABS (which stabilizes the car unrealistically when braking) but it's certainly worth it to learn it. Without ABS it certainly plays a bigger role since you lock your brakes easier if you upset the car with engine braking.
When I first started racing with a clutch, when I was approaching a braking zone I always hit the clutch and grabbed the gearstick first before braking, which is a mistake. It's important to first move your right leg to the brake and after you've braked a fraction of a second, then kick the clutch while using the outside of your right foot to simultaneously touch the throttle a bit.
After I figured out the order of the actions I needed to perform, it didn't take but a day to learn it and soon after that it started to come natural.
Practice by building up to a high speed, and then braking to 0 while shifting just once and try to heel-toe that shift. Remember to move your right foot to the brake first, and then while you're decelerating try to do shift down while touching the throttle with the outside of your foot. It's alright to let the brake pedal rise a bit from the bottom while you blip the throttle (you shouldn't have it down all the way anyway unless you're using ABS) and then just release the clutch.
Next time try shifting down twice, then thrice, and so you'll eventually learn to do it so fast you can shift all the way from 6th to 1st and it'll be memorized in your muscle memory.
