gap insurance covers the shortfall that may occur in instances that the car is wrote off in a smash, as you've paid cash there would be no shortfall on the money paid to the finance company.
Look closely at your existing insurance details. Some will replace your car with a brand new one (same model of course) if you write it off within 12 months. So you will have no need for gap insurance.
easy way to look is in 6 months you're cars wrote off (imagine on finance) and the insurance pay £9.500, this leaves you short of £3,500... this is what the gap will cover (the £3,500 shortfall) but has you've paid cash there is nothing really to be gained by having gap insurance.
vehicle replacement works the same as the return to invoice which i've tried to explain.
check this site as it will explain in pictures also how it works.
RTI will give you price you paid for the car (ie the invoice price) - but Vehicle Replacement gives you the cost of replacing the car. If you'd had the car, for example, a couple of years before it was written off, the cost of replacing it would have gone up.