1) Book an appointment. Do not be fobbed off with the line about "if DWP want evidence they will ask me". Some will attempt to charge. Consider in that case if it's worth it. It can often be better to simply ask for your entire medical records. £10 charge.
2) Ask yourself what the weak points in your evidence are. Will GP evidence fill those gaps?
3) 52% of those PIP claims which succeed do so without any additional evidence. There's a reason for that.
Paying is a personal choice as you say but many people on a fixed income either don't have that choice or make the payment because they are desperate and feel they have no choice. Tribunals often adjourn for relevant parts of medical records anyway; GP evidence is largely irrelevant if you're already referred to a specialist. It's easy enough to be duped into thinking something has a value beyond that which it actually has. It's why I have forcibly made the point elsewhere that the value in recorded delivery is minimal when compared to the fact you can email stuff in for nothing and it's not appropriate or accurate for advisers to suggest that it has that value for people on fixed incomes.