Why do some words with similar meanings sound similar as well?

Yes, all of your examples share the same etymology and can be traced back to the PIE root *ghel. As explained in etymonline.com (emphasis mine):

*ghel- (2)

Proto-Indo-European root meaning "to shine;" it forms words for "gold" (the "bright" metal), words denoting colors, especially "yellow" and "green," also "bile, gall," for is color, and a large group of Germanic gl- words having to do with shining and glittering and, perhaps, sliding. Buck says the interchange of words for yellow and green is "perhaps because they were applied to vegetation like grass, cereals, etc., which changed from green to yellow."

It forms all or part of: arsenic; Chloe; chloral; chloride; chlorinate; chlorine; chloro-; chloroform; chlorophyll; chloroplast; cholecyst; choler; cholera; choleric; cholesterol; cholinergic; Cloris; gall (n.1) "bile, liver secretion;" gild; glad; glance; glare; glass; glaze; glazier; gleam; glee; glib; glide; glimmer; glimpse; glint; glissade; glisten; glister; glitch; glitter; glitzy; gloaming; gloat; gloss (n.1) "glistening smoothness, luster;" glow; glower; gold; guilder; jaundice; melancholic; melancholy; yellow; zloty.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit harih "yellow, tawny yellow," hiranyam "gold;" Avestan zari "yellow;" Old Persian daraniya-, Avestan zaranya- "gold;" Greek khlōros "greenish-yellow color," kholos "bile, gall, wrath;" Latin helvus "yellowish, bay," Gallo-Latin gilvus "light bay;" Lithuanian geltonas "yellow;" Old Church Slavonic zlutu, Polish żółty, Russian zeltyj "yellow;" Latin galbus "greenish-yellow," fellis "bile, gall;" Lithuanian žalias "green," želvas "greenish," tulžis "bile;" Old Church Slavonic zelenu, Polish zielony, Russian zelenyj "green;" Old Irish glass, Welsh and Breton glas "green," also "gray, blue;" Old English galla "gall, bile," geolu, geolwe, German gelb, Old Norse gulr "yellow;" Old Church Slavonic zlato, Russian zoloto, Old English gold, Gothic gulþ "gold;" Old English glæs "glass; a glass vessel."

The same principle holds for repugnant and repulsive both of which are derived from Latin words using the word forming element -re meaning "back to the original place; again, anew, once more". In the case of repugnant, the etymology (according to etymonline) is (emphasis mine):

repugnant (adj.)

late 14c., "contrary, contradictory," from Old French repugnant "contradictory, opposing" or directly from Latin repugnantem (nominative repugnans), present participle of repugnare "to resist, fight back, oppose; disagree, be incompatible," from re- "back" (see re-) + pugnare "to fight" (from PIE root *peuk- "to prick"). Meaning "distasteful, objectionable" is from 1777.

And that of repulsive is:

repulsive (adj.)

early 15c., "able to repel," from Middle French repulsif (14c.) and directly from Medieval Latin repulsivus, from repuls-, past participle stem of repellere (see repel). The sense of "causing disgust" is first recorded 1816. Related: Repulsively; repulsiveness.

While repel's is (emphasis mine):

repel (v.)

early 15c., "to drive away, remove," from Old French repeller or directly from Latin repellere "to drive back," from re- "back" (see re-) + pellere "to drive, strike" (from PIE root *pel- (5) "to thrust, strike, drive"). Meaning "to affect (a person) with distaste or aversion" is from 1817. Related: Repelled; repelling.

As you can see, both words can be traced back to the same -re.

So yes, for both the sets you mention in your question, the reason they sound similar is that they share similar etymologies. I believe it is this shared origin which gives rise to the phonaesthetic effect described in Decapitated Soul's answer. The reason these words all sound similar is because of their shared etymology, and the result of this similarity is the phonaesthetic effect.