I don't think there are barriers at Victoria Coach Station, it is not that sort of place. You shouldn't have any problem getting someone to point you to the right arrival slot.

The only obvious issue is London traffic, and the coach could arrive substantially earlier or later than the stated time depending on traffic conditions. From the coach station it is a very short walk across the road to the railway station.

However ... from Liverpool, the potential for the coach to be substantially late is high as there is a lot of road to cover and a lot of potential for accidents etc to close the motorway. This could be an issue given you are going for a plane. The train is more predictable. Liverpool trains go into Euston, it is then a 10 minute walk down the road to Kings Cross to get the train to Gatwick.