Hey there

I find myself invariably incapable of picking up on others' ideas and coming up with a list myself. As regards your word clusters, a mundane explanation would consist, I suppose, of two reasons:

1. Plain simple onomatopoeia.

2. Etymology - if the root word (say in PIE) had a 'gl-' sound, then all variants, since they would be its descendants, would have the same sound.

As far as adding to the list is concerned, the sw- sound seems to be associatd a lot with flowing/moving words: swish, sweep, swoop, swipe, swarm (the verb), swan (v.) etc

cheer

the sunshine warrior