There’s nothing incorrect about Gail’s French.
In the first sentence, replacing ‘en’ with ‘y’ changes the meaning (from “the elephants: the plains are full of them” to “the elephants: the plains there are full”) and both are correct grammatically.
In the second sentence, changing the verb also changes the meaning, from “Thanks to G-d, the elephants are completely gone” to “Thanks to G-d, the elephants have completely disappeared.” Both are grammatical, but the meaning and the tense of the verb change. Ordinarily, the verb ‘disparaitre’ takes ‘avoir’ as an auxiliary verb, but it’s perfectly legitimate to use the verb ‘etre’ with a participle as a predicate adjective.
And replying to the original question: ‘the’ is an adjective that makes no grammatical sense without a following substantive. Unless you’re using it to refer to the word specifically, and not in a grammatical function, then no, you can’t end a sentence with ‘the.’ Unless, of course, you’re creating some kind of word salad experimental poetry, in which case all bets are off.