As we've got a few more faces racing who are new to the game / series, I thought I'd do a small update to this.
Firstly, a couple tips about the HUD
1. Gear indicator
This is the gear the game thinks you should be using for the next corner and will flash red when the game thinks you need to brake for the corner. In my experience, it's too conservative and will always tell you to brake too early and mostly to use a gear 1 lower than actual. However, it acts as a good starting point if it's a track you don't know and as you get more familiar you can start pushing your braking point later
2. Throttle position
GT7, much like GT Sport, the lower the Traction control (TCS) you can use the faster you'll be. Though it's always important to remember, lower TCS will save you 1-2 seconds a lap but set it too low and spin, that'll cost you more like 20-30 seconds. You can however do a little practise of using lower TCS as the throttle position goes white, as in the screenshot, to show you're at 100% throttle but if you engage TCS, a portion at the top will turn red to show how much the game is limiting your throttle. If out of every corner you turn it red, try to be a little more gentle and ease on to the throttle. If you never turn it red, try turning down your TCS. It's not quite that simple as at lower levels TCS cuts in later so you can be more aggressive with the throttle before it activates but it's a small part of the picture of if you're over-driving the car.
3. Driving assists
Which nicely leads on to driving assists. I think Phil mostly disable these on the server due to a certain Mike incident. However, much like TCS, they turn red when the assist is active and again, like TCS, activation of an assist will generally cost you time in GT7 (ABS it the exception to this rule*). So again, if you're constantly activating these over a lap, try being smoother, easing the stick to steer rather than 0 - 100% steering instantly etc.
*ABS is the exception to this rule - technically speaking ABS does also slow you down too BUT it's incredibly hard to threshold brake without locking up. Lock up - and you'll be slower. Don't brake quite as hard to avoid locking up - you'll be slower. There's a tiny tiny margin in the middle where braking without ABS is faster. So keeping it ABS at default will overall make you faster as you'll be far more consistent.
Smoothness
I've mentioned this a couple times and I want to explain some of the theory behind it - Slow is smooth, smooth is fast. Often times you'll a lap where you're on the edge and you have to really work the car will be slow and the laps that feel slow will be your fastest. Keeping the car balanced, with little to no understeer or oversteer, will mostly feel slower but is considerably faster. Now the question is how to achieve this smooth is fast - this can be broken down in to the 3 areas of a corner; braking, turning and accelerating. In a perfect world, each should only be done once - brake before the corner and come off the brakes as you turn in, turning to the maximum lock required for the corner at the apex, then start opening the steering as you feed on to the throttle. If you needed to get back on the brakes then you braked too late, if you get on the throttle before the apex you braked too early, if you need to change steering input you turned in too early or too late, if you need to counter steer on corner exit you got on the throttle too early or too sharply.
I'm not saying you should be thinking all this all the time but it's something to keep in your mind while practising or watching a replay of your laps. As you go round the lap, make a mental note of things such as braking too early for turn 3 and too late for turn 9. Then the next lap, make small steps to correct these on the next lap while making mental notes of thing for next lap. Basically small incremental changes each lap can equal a decent improvement on overall lap time.