let me unpopular.

my answer is.............no.
this choice is not tied to KG's career performance, heart, abilities, and the like. the point here is not whether KG was great player. he clearly was. the question i ask is whether among the pantheon of celtics players, has KG accomplished enough
as a celtic to be listed among the all time celtic greats.
ability, longevity, attitude, celtic-devotion....many of these are not measurable, yet i believe all of them may be used in making a decision about whether a player's number should be retired.
to have a number retired on the celtics is something that is exceptional and clearly out of the ordinary on many counts, and i agree that a high standard should be invoked. popularity alone is not enough.
one championship, in celtic folklore, does not mean greatness, it is expected. so on this point, KG passes but a single championship does not place him on a separate or elevated plateau above other celtic greats. being a celtic fan, i rejoice and an happy he helped the celtics win that championship, but, it does strike me that it is a single championship. if KG had been healthy and lead the celtics to multiple championships, then my answer might be "yes, retire #5 today."
but he wasnt and he didnt.
next, while in boston, KG was an outstanding player. but how long was he a celtic and of that time, how long was he injured, reducing his impact and play as a celtic?
KG played only 396 games in 6 seasons for the celtics.
for reference purposes, player/games of the top ten celtics:
1. John Havlicek* 1270
2. Robert Parish* 1106
3. Paul Pierce 1102
4. Kevin McHale* 971
5. Bill Russell* 963
6. Bob Cousy* 917
7. Tom Sanders 916
8. Larry Bird* 897
9. Don Nelson 872
10. Sam Jones* 871
while a celtic (and only celtic numbers count for retiring a number) KG averaged roughly:
15.7 points a game
8.3 rebound a game
2.7 assists
all of these are well below his pre-celtic averages. these celtic numbers are good numbers, but not HOF, retire-that-number-now numbers. that is, as a celtic his stats were good, but not great.
but, as observed in this thread, his heart and desire and passion were always off the charts, numbers be [dang]ed. and i agree to a degree. but even if we allow that "heart" to be one determining factor and ignore the good numbers i still have a central reservation.
he wasnt a celtic for very long in his career.
longevity as a celtic is important to me. here is where KG is clearly lacking. six seasons out of a 19 season career means, to me, that for all his skill and heart and effort, 2/3 of his playing days were elsewhere, not in boston. to have your number retired, you should be a member of the boston celtics for the majority of your career. now i admit, winning a bundle of championships can serve as a substitute, but as discussed above he only won one championship with the celtics.
i loved KG as a celtic and he made a mistake early in his career by not leaving minnesota as a free agent to join the celtics. the two were almost fated to meet. but retire his number? no. as wonderful a player as he was, he does not meet the standards set for the celtics to have his number retired.
and by the way, this is an excellent thread and has raised a number of thoughtful and interesting posts. i extend my thanks to everyone.