<p>"long since" is an adverbial expression that means "for a long time." its use here is proper. i don't think you're likely to see it again when you take the SAT, but, if you do, remember that this usage is fine. also remember this one, by the great irish poet kavanagh:</p>
<p>On Pembroke Road look out for my ghost,
Dishevelled, with shoes untied,
Playing through the railings with little children,
Whose children have long since died.</p>
<p>another morbid sentence, and another correct use of "long since" (here, "long since" is used to modify the verb phrase "have died," as opposed to its use in the CB sentence to modify the adjective phrase "gone"). i never miss a chance to quote an irish poet :)</p>