Provenance
Where it comes from.
'Dental' comes through French and Medieval Latin dentalis from Latin dens (genitive dentis), 'tooth,' and ultimately the Proto-Indo-European root dent-, 'tooth,' the same ancestor behind Sanskrit danta and Greek odontos. English adopted it in the 1590s, narrowing toward 'used in dentistry' by the 1820s.
'Gain' arrived around 1200 from Old French gain, 'profit, advantage,' with deeper roots in a Proto-Germanic word for hunting and foraging, the pursuit of something worth having. The pairing lets a very old word for the tooth carry a very old word for reward.