You can get train from Kings Cross to London Gatwick on that day you want for £14.50 (Duration: 1hr 32mins) and back from London Gatwick to Kings Cross for £10 (Duration: 1hr 2mins) on the 11th July.

From St Pancras to London Gatwick it will cost £10 going (Duration: 1hr 12mins) and coming back from London Gatwick to St Pancras, it'll cost £10 on the 11th July (Duration: 46mins)

Hope this what you mean