Not particularly, there is perhaps an argument for a very subtle nuance between Gae and Gost, as Gae comes from an original root meaning 'fear' and Gost comes from a root that had a slightly stronger 'terror' meaning. Other than that, they're just the results of different original roots
There's no difference. în and idhrinn both mean 'year' in the sense of a 'sun-round', i.e. what we call a year, one orbital period of the Earth around the Sun. A Valian year is 144 of our years, Ennin means literally 'long year'. Pretty much the equivalent of a century to us (the Elves worked in base 12).
Fir- = to die, used of mortals who actually die. Gwanna- = to depart, to die, used of Elves who never actually leave the circles of the world regardless of whether their physical form falls in battle or not (as they get reborn at some point after the Halls of Mandos). Also used to refer to Elves who depart from Middle Earth via a ship to Valinor.
Pel- = to fade, to wane (to dissipate). Gwanna- = to fade, in the sense that the Elves grow weary of Middle Earth.