It can be a very fine balance. Some factors to consider before getting to GT settings:
Are you using solid boosters? GT can't throttle those, so once you lose the TWR the boosters give you, and if the remaining engines have low TWR, your ship will pitch up hard to keep the apoapse time in line with what was set. This severely impacts efficiency, which may lead to not reaching orbit with the fuel you have left, or just immediately flipping. The boosters may also push you to too high a speed at too low an orbit, if you set a higher launch angle.
Some components have a lower max temp than others. Use the service bay or fairings to protect these
Here's the practical impact of each setting on your launch profile, assuming you only change one setting, leaving the others:
Start m/s: Larger values will increase vector and gravity drag. Your ship will be at a slightly higher altitude when it first aligns with the orbital vector, reducing heating a little. In general, smaller values are more aggressive and efficient, but increase the time spent lower in the atmosphere.
Turn Angle: Smaller values will increase vector and gravity drag but reduce odds aerodynamic flip for unstable ships. Your ship will be at a slightly higher altitude when it aligns with the orbital vector, reducing heating a little.
Hold AP Time Start/Finish: How far ahead in seconds your apoapsis is pushed before engines throttle back. Larger values at start time will increase vector and gravity drag, and decrease time spent at lower altitudes. Finish time mainly has an impact on how your AP Time changes up until it gets overridden in effect by Sensitivity.
Sensitivity: The minimum thrust in percent. Overrides Hold AP Time because thrust won't be reduced below this level when the ship is trying to keep time-to-apoapse at a certain value. Higher values will force your ship to reach Destination Height sooner, reducing time spent in atmosphere.
Destination Height: A higher value here can cause your ship to accelerate more in atmosphere once you are aligned with the orbital vector, increasing heating. Very low sensitivity values offset this but increase time spent in atmosphere.
Pressure Cutoff: A lower value will increase how long your ship waits to switch from the Surface prograde vector to the Orbital prograde vector, increasing vector drag but reducing flip chance for unstable vessels. 2500 is roughly 20km; if this is where your ship suddenly turns down to the horizon and flips, it's because of this setting.
Setup options:
Fairing Pressure is set by default to much lower in the atmosphere (apparently?) than the Pressure Cutoff. There are a couple things you can do to keep your fairings on higher in the atmosphere, and setting this to a lower value is one of them. Note that GT might not pop your fairings off when they're staged due to this, so it is possible for your ship to be stuck inside a dropped stage's fairing. Be ready to manually release the fairings when you feel it's safe, heat-wise, just remember you can't do that if the fairings were in a stage you just dropped. The resource scanner dish, for example, has to be protected by fairings much longer than the default settings will, so reduce Fairing Pressure (maybe to 0) and manually decide when to let them go (usually over 55k).
The default settings shouldn't generally cause problems, particularly for early career.
Depending on your ship, Pressure Cutoff might cause a flip, but otherwise we'll need to see what your ship looks like and when it burns up to figure out what GT settings might need to change.
