The problem at Euston is that the concourse already gets crowded, and you need to maintain access to facilities like the toilets.
I doubt you could put gates on the ramps themselves. In any case gates are arguably not suitable for long distance services.
Avanti trains should really be doing their ticket checks done onboard, although I don’t see anything inherently wrong with a manual check before boarding, as long as this is done in a customer friendly manner.
LNR from ungated platforms are more of an issue, again all of these trains could have ticket checks done onboard if there was a will.