終幕 literally means “the final curtain” or “closing act,” and is a poetic word often associated with a dramatic ending rather than an ordinary conclusion.
In GAIA, the phrase 終幕を知らない母なる海 describes the “mother sea” that knows no ending, giving the song a powerful image of nature as eternal and limitless.
"GAIA" is Janne Da Arc talking straight to the planet. Almost every line ends with よ (yo), the particle you use when you call out to someone, so the singer is addressing the sky, the earth, the sea and the wind one by one: 鳥さえ届かない完璧な空よ (O perfect sky that even birds can't reach), 終幕を知らない母なる海よ (O mother sea that knows no final curtain). It is a roll call of nature, and each one gets a big, slightly over-the-top title. The earth is the one that 繰り返す破壊も恐れない (isn't afraid of destruction, again and again), which is a nice way of saying it has survived everything so far.
In the middle of all this the singer keeps asking 今何を必要とする (what is needed now?) and then answers with his own arrival: 今ここに僕は生まれる (here and now, I am born). By the end he is asking the planet itself, 限りある命を与えた GAIA よ (O GAIA, who gave us a life with limits), to bring his fading voice back: 再び命を吹き込んで (breathe life into it again). The "La la la" lines are not filler; each one leads into a real phrase like 空を越えて (beyond the sky) or 癒しの海 (the healing sea). Learner tip: 唄う (to sing) here uses an older, more poetic kanji than the everyday 歌う.