Sorry to anyone if this is just a repeat of any prev info regarding shifting, it's the way I like to shift.
To get good performance from your car
find out at what rpm your car is making it's peak power at and change around then, you need not cain the engine into the redline if it's actually gutless for 300-400 rpm and wants to change up before then.
As for braking using gears, I find I
brake first then down change accordingly,
changing down too fast from a high speed or bloc shifting will make you loose traction, basicly if you are bouncing off the limiter every time you change down you are doing it wrong imo, however you can use it to your advantage when you want to break or regain traction for what ever reason, it wears your tyres out quicker though.
Rev matching, I prefer mt to at because, similar to irl, I can rev match up and down the box so that I can maintain the optimum revs for a hill or sharp bend, infact
I wish there was a clutch in GT4 because in over 10 years of rl driving I like to heel and toe when down shifting (to save wear mostly, and for a smoother drive) this is one advantage of being taught to drive in a 60's moggy minor with unresponsive syncromesh and non in first!
You probably already know this but gear changes and throttle control go hand in hand;
It's good practice to learn throttle control because you may want to hold the revs (say half throttle) but not change up or down gear.
To see how much of an effect being able to feather the throttle makes try unplugging the wheel etc or ps2 pad and putting an old grey PS1 controller in, they have no pressure sensitivity on the buttons, then
try driving a modified AC Cobra, it's not fun only having full brake or throttle pedal travel available, you'll be doing donuts on every hairpin.
Learn your own shifting style; when you get used to some tracks the shift hint you get flashing in red can be disregarded, I often take corners in a higher gear and at a greater speed than the guide is telling me to because I am confident about nailing the car and using every inch of the track if nothing is in my way, it's a good feeling screaming around a corner in 4th when the gear hint is flashing 3rd at you.
I find MT really rewarding and after time you will too. Good luck.