I have always been confused by the entrustment of the earth to Adam. Is the earth a gift, or do we merely have stewardship? Do any rabbbanim state an opinion, particularly whether any Acharonim weigh in on the conservation and environmentalism?
The Midrash (Koheles 7:28, cited by Messilas Yesharim chapter 1) writes (translation mine)
When God created Adam he took him to see all the trees of the Garden of Eden and told him "Look at my creations; how beautiful and praiseworthy they are. All that I have created, I created [only] for you. Be careful not to ruin and destroy my world."
It appears that the world belongs to God and we are required to watch over it and not destroy it.
Note: This Midrash is cited by Messilas Yesharim in the context of not destroying the world as a result of our sins and lack of attention to the Torah. To interpret it in the context of environmentalism is probably an anachronistic interpretation. Only if the environmentalism is proscribed by and consistent with the Torah (and certainly not contradicted by explicit required mitzvas) will this source be traditionally and legitimately applicable to environmentalism.
I can't say if the planet Earth, itself is a gift. It's unclear from your question what you are referring to. However, Talmud Brachot 35a resolves 2 seemingly opposing verses from Tehillim (Psalms) - "The earth and its whole are God's" and the verse "The heavens are heavens to G-d and the earth he gave to the children of man." In resolving this, the Talmud explains that things coming from the earth (produce, e.g.) belong to G-d and become man's only after blessing G-d for these things. From this sequence / explanation, I would infer that things from the earth are given to us as a gift.
וְהָאָרֶץ, לֹא תִמָּכֵר לִצְמִתֻת--כִּי-לִי, הָאָרֶץ: כִּי-גֵרִים וְתוֹשָׁבִים אַתֶּם, עִמָּדִי. (Vayikrah 25:23)
From this pasuk, it seems that we are merely taking care of it, but the Earth still belongs to Hashem.
there's no free lunch. we are all just workers here and must use the earth in fulfilling our work, not however we want.
Genesis 2:15 "The LORD God took the man and put him in the Garden of Eden to work it and take guard it." The midrash raba there expounds: "work it" refers to performing the positive commandments and "guard it" refers to guarding the negative commandments.
Man was created to serve God (even though God does not actually need anything)
"For man is born to toil..."(Job 5:7)
see the book Chovos Halevavos especially gates 2 and 3 for an explanation.