1. PIE root *ghel- "shine, glitter, glow" => Proto-Germanic *glim- => gleam, glimmer, glimpse.
2. glimmer is a frequentative from Proto-Germanic *glim-, originally "shine brightly," Sense shifted 15c. to "shine faintly".
3. PIE root *ghel- "shine, glitter, glow" => glow, glitter, glister, glisten, glint, glad.
4. glow + -ing.