If one buys a 1 to 3 day london pass with travel card would that cover a daily return train trip from Horley Station(near Gatwick Airport)to the centre of London
No, Horley's too far out. You could get a ticket to Coulsdon South, and use the travel card from there.
The service from Horley to Coulsdon South is hourly. Some more details here
http://en.wikipedia.org/wiki/Coulsdon_South_railway_station
Alan