I would, at least. While I took MI1 (and even 2) quite literal when I played them as a kid, I now believe that Guybrush is a kid at a (possibly pirate-themed) theme park, indulging in his fantasies. LeChuck must be his older brother (we all know, from hearsay or experience, that older siblings are always mean towards the young ones, right?).
Anyway, to answer the question, Guybrush just entered the park through the main gate (represented by the stone arch).
But who knows, maybe he’s just a poor guy tied to a chair somewhere in a lab, subjected to cruel brain experiments that cause funny “dreams”.