Gareth sits next to 3 phonetic neighbors — Garrett, Garth, and Garreth are the closest matches by sound. Most share Gareth's Welsh cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"gentle, gentleness"
"son of the right hand"
"short form of Matthew"
"ridge of rock or coral beneath the sea"
"immortal, divine"
"bright army, famous warrior"
"sword"
"beloved, dear one"
"winner, victorious fighter"
"God has remembered"
"hilltop settlement of the friend"
"hill, marshland"
"one who drives or operates wheels"
"one who resembles God"
"strong lord, chief"
"grey, rocky land"
"rock (from city of Tyre)"
"son of grace; graceful"
"stone of help"
"flower, flourishing"
"army man, soldier"
Similar sound: These names share phonetic patterns with Gareth — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Welsh origins, giving them the same cultural heritage as Gareth.
Rhythm: Gareth is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.