One more gloss:

Mulapannasa-Atthakatha 2.386

Sugataati su.t.thugataa suppa.tipannaa kaarakaa yuttapayuttaa.
Evaruupaapi hi ekacce bhikkhuu atthi. Duggataati du.t.thugataa
duppa.tipannaa kaayada.lhibahulaa vissa.t.thakamma.t.thaanaa.
Evaruupaapi ekacce atthi.

"Well-gone, intent on going (practising) well (by the good way)."

However su.t.thu is a usual commentarial explanation of the prefix su-.

Robert Eddison wrote:
RE> Buddhaghosa gives four glosses on Sugata in his Visuddhimagga:

RE> He is called Sugata...
RE> sobhana-gamanattaa -- because of a beautiful manner of going.
RE> sundara.m .thaana.m gatattaa -- because of having gone to a beautiful place.
RE> sammaa gatattaa -- because of having gone rightly.
RE> sammaa gadattaa -- because of enunciating rightly.

RE> (For a fuller treatment of these see Path of Purification VII 33)

RE> and a similar set of four in his commentaries to the Khuddakapaa.tha and
RE> Suttanipaata:

RE> sobhanena gamanena yuttattaa -- because of being conjoined to a beautiful
RE> manner of going.
RE> sobhana.m .thaana.m gatattaa -- because of having gone to a beautiful place.
RE> su.t.thu gatattaa -- because of having gone the best [way].
RE> su.t.thu eva gadattaa -- because of enuciating only the best.