People seem to be under the impression that expanding Gatwick will not cause any increase in traffic on the M25/M23. Also the rail station was recently crowned the worst in the UK. Do you really not believe there will be significant works required after this has been completed?
Just because it is not being included in the project itself as LHR has to be, doesn't mean the infrastructure around there won't need it in the future as the airport grows.
The transport works already planned or being built to LHR will be adequate for years.