I don't think every station will need to gated full time. Existing smartcard systems such as TfL's Oyster have many stations ungated and only have smartcard validators at those stations. Others such as Nexus's POP card only have a handful of gated stations (and those are only staffed part-time), the rest only have validators - and unlike Oyster, POP season ticket holders must touch in and touch out every time.
Part fo the problem is that in many parts of the country, the same train performs multiple functions: it might be providing InterCity, regional express and even local journeys at different points along the route. Whilst PAYG is perfect for local journeys, it's unlikely that people would want to use PAYG for InterCity journeys. It would be better if people knew that PAYG was available for 'local' trains, but for InterCity trains you have to buy your ticket from a ticket machine before boarding - loading the ticket onto your smartcard. But defining what is a local train, what is a regional express, (what is an inter-regional express) and what is InterCity would be a bit of a nightmare!
That is very true!