Of the highest quality (3-5)
(Other definitions for gilt-edged that I've seen before include "Of high quality and reliable" , "Like reliable stocks or securities" , "Of investments, considered to very safe" , "(Of investments) regarded as extremely reliable" .)
'of the highest quality' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for top-notch that I've seen before include "Excellent (informal)" , "Grade 1" , "Of the highest quality" , "First-rate" , "Superb" .)