It is near Toussaints
It is near Toussaints, the living and dead will say:
‘Have they ended it? What has happened to Gurney?’
And along the leaf-strewn roads of France many brown shades
Will go, recalling singing, and a comrade for whom also they
Had hoped well…
On the night of all the dead, they will remember me,
Pray Michael, Nicholas, Maries lost in Novembery
River-mist in the old City of our dear love, and batter
At doors about the farms crying ‘Our war poet is lost.
Madame, no bon!’ – and cry his two names, warningly, sombrely.
Ivor Gurney
The poems of the First World War poet Ivor Gurney dance on the edge of disintegration – it could be argued that not many of them achieve completeness, but they are brave and vulnerable and usually offer some memorable quirk of observation or language.
David Sutton 
“La Toussaint, 1st November, combines All Saints Day and All Souls Day as a jour ferié, or public holiday. Families attend an All Saints Mass in honour of the Catholic saints and of dead relatives.” I’m guessing that this poem was written in 1922 or later. Neither the living nor the dead can find Gurney because he’s imprisoned in a psychiatric hospital?
I believe you are right, that this was a poem written after his incarceration in the psychiatric hospital, though I don’t know exactly when.