Welcome to the forum!
As
@30907 has said, e-tickets don't have to be scanned or activated in any way in order to be valid. Therefore there's no need to do anything at Coulsdon South to make your e-ticket valid.
It's also worth noting that when travelling beyond the area where a Freedom Pass is valid, you don't need to stop at the station where you switch to your 'add-on' ticket. You just need to go via the station where you switch from one ticket to the next - whether you stop there or not.
Therefore, you could take a train that runs non-stop through Coulsdon South. And although trains that run non-stop from East Croydon to Gatwick are usually planned to use the "Quarry Lines" which technically speaking don't pass through Coulsdon South, they pass by very close to the station and are therefore generally still considered to be running through the station for ticketing purposes.
In future, I would suggest buying a ticket from East Croydon or Boundary Zone 6 to Gatwick. Either costs the same as a ticket from Coulsdon South - and as all Gatwick-London trains pass through East Croydon this is unambiguously valid on all trains from Gatwick to London. Boundary Zone 6 is a conceptual location used if you want to extend the validity of a Zones 1-6 ticket (such as your Freedom Pass), and therefore a ticket to/from a Boundary Zone location doesn't even require you to go via any particular station.
Buying a Boundary Zone ticket doesn't really make much difference for a journey to Gatwick as all trains go via East Croydon - but for journeys to places such as Dartford, where there are different boundary stations depending on the route you take, it can make a difference and so it's worth buying a Boundary Zone ticket as this usually costs the same (if not less) than a ticket from the last Zone 6 station, whilst giving you more flexibility.
East Croydon tickets are of course easy to buy at any app or website that sells train tickets. Boundary Zone tickets are only sold at ticket offices, some train companies' ticket machines, and on a few websites such as Avanti, c2c and the forum's own ticket site (you can access this via the "Tickets" button on the top menu of the page - you need to specify that you have a Freedom Pass in the advanced options).