Dear Matthew,
I do think your Gradus ad Parnassum must have played a role in Nabokov's choosing this name - - and names are clues with flags on in this novel.
If I am correct and the fugue as 1) theme of pursuit/chase and 2) psychological state are appropriate in Pale Fire, it is more than likely that Nabokov also had the musical meaning in mind. Since Gradus is the ultimate pursuer (fuguer? fuguest?) it makes particular sense.
It could be another coincidence but I vote for authorial intention here.
Carolyn
p.s. I don't comment on the texture/text bit because it has always struck me as either fatuous, obvious or something else I can't quite make out.