Glint, glisten, glitter, gleam...
Tiffany thought a lot about words, in the long hours of churning butter. 'Onomatopoeic', she'd discovered in the dictionary, meant words that sounded like the noise of the thing they were describing, like 'cuckoo'. But she thought there should be a word meaning 'a word that sounds like the noise a thing would make if that thing made a noise even though, actually, it doesn't, but would if it did.'
Glint, for example. If light made a noise as it reflected off a distant window, it'd go 'glint!' And the light of tinsel, all those little glints chiming together, would make a noise like 'glitterglitter'. 'Gleam' was a clean, smooth noise from a surface that intended to shine all day. And 'glisten' was the soft, almost greasy sound of something rich and oily.
I think the word Tiffany is looking for is phonestheme or maybe ideophone (more here). I think Pratchett's view of the impression these gl- words give is spot on. I wonder how much that has to do with being a native English speaker. The Japanese ideophone for sparkling or glittering is キラキラ kira kira - a similar sound combination - velar stop plus lateral - used to represent the same symbol. [I know: most likely a coincidence. See words like click, clip, clink, as Paul D. astutely points out.] On the other hand, the Japanese ideophone uja uja represents "many small things gathered together and moving, such as a swarm of insects or a crowd of people seen from a distance" - not my first guess of what uja uja would mean.
The gl- phonestheme seems to be common to the Germanic languages. Here's a list of the English gl- words, all having something to do with light, with some cognates. This is all from the OED unless noted. According to the American Heritage Dictionary of Indo-European Roots, all these words are from *ǵʰel- "to shine", the same source as gold and yellow.
glad from Old English glæd "shining, joyful", cf. Old Norse glaðr "bright, joyous", German glatt "smooth"
glance "any of various minerals that have a brilliant luster", from Old High German glanz "bright" (AHD)
glare from Middle English glaren "to glitter", from Middle Dutch glaren "to gleam, glare"
glass cf. German Glas, Old Norse gler, Middle Low German glār "amber"
gleam cf. Old High German gleimo "glow-worm"
glee from Old English glēo "sport, merriment", cf. Old Norse glý
gleed cf. Dutch gloed "ember"
gleg from Old Norse glöggr "clear-sighted" (AHD)
glimmer cf. Swedish glimra "to glimmer"
glimpse cf. Middle High German glimsen "to gleam"
glint cf. Swedish glinta "to slip, slide, gleam"
glisten from Old English glisnian, cf. Middle Low German glisen
glister from Middle Low German glistern
glitter from Old Norse glitra "to shine", cf. German glitzern "to sparkle"
glitz from Old High German glīzen "to sparkle" (AHD)
gloaming from Old English glōm "twilight" (AHD)
gloat cf. Old Norse glotta "to grin"
glogg from Old Norse gloð "ember" (AHD)
gloss "surface shininess or luster", cf. Icelandic glossi "blaze"
glow cf. German glühen, Dutch gloeien, Old Norse glóa
glower perhaps from Low German glōren "to gleam, stare", cf. Icelandic glora "to gleam, stare"