@Emzinhoo. Noted what you say.
An undiscounted (one way) Anytime Day Single from East Croydon to Gatwick Airport is £6-20.
You may just get asked to pay this amount plus the £150 fee that
@Hadders has mentioned upthread.
However, if the rail company's investigation team check through your ticket purchasing history more diligently (easy enough for them to do, if the inappropriately discounted tickets have all been bought online using the likes of Trainline) the amount they will seek will soon add up once they realise you've been underpaying for some time.
So, potentially 2 x £6.20 for a daily return journey, multiplied by the number of times that the journey has been made per week, multiplied by 20+ weeks.
And no allowance made for amounts paid already for incorrect tickets. You'll also be expected to pay the total amount payable in a single, lump sum.