Whenever you gain life, put a +1/+1 counter on Trelasarra and scry 1. (Look at the top card of your library. You may put that card on the bottom.)
(23/07/2021)
If you gain life at the same time Trelasarra, Moon Dancer is dealt lethal damage, it dies before its triggered ability can put a +1/+1 counter on it, but you will still scry 1.
(23/07/2021)
Each creature with lifelink dealing combat damage causes a separate life gain event. For example, if two creatures you control with lifelink deal combat damage at the same time, a "whenever you gain life" ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.
(23/07/2021)
An ability that triggers "whenever you gain life" triggers just once for each life gain event, no matter how much life you gain.
