It's in the B check-in area prior to security (landside). Although in practice I imagine you can use any check-in desk if quicker.
There is no 'gate security' at T5. Do you mean at flight connections, if you are flying in from elsewhere?
Note you are not obliged to take up this offer. I wouldn't and don't. If in CE, as long as your bag is within the allowed dimensions and you're not boarding late, you'll be able to take it onboard.