What is it that you don't understand about making gears in Construct2?
I guess you should have these variables:
Than you should create some formula's that test these variables in relation to eachother.
engine speed is probably the one that is most important in this formula, for it is the one that is more limited in higher gears.
higher gears need less engine power to keep speed, but more engine power to accelerate.
It will make the difference between choosing higher and lower gears in the hills.