POV: Your alien girlfriend wakes you up in the middle of the night.

Basically, you use ritual casting to create a 150ft cubic space that looks like black fog from the outside. On the inside, it's whatever the caster wants it to be. A private apartment in a Waterdeep Sea Ward tower, for example. Think like a TARDIS for space relevance as well.

My TTRPG caster who has this spell uses it for long rests where the party really doesn't want to be disturbed. A plain campsite is usually fine, but for extensive healing or planning, the expense of casting Private Sanctum is worth it.

For Gale, the privacy is worth it. If he's feeling homesick, he would likely cast it for that purpose on the reg (and I believe he does, provided he has the components).

That would also explain how you can change among the tower, the forest, the Astral Plain, etc. You're not really in Waterdeep; he shows you that it's a spell when he chooses the setting of the Sanctum and shows that he can change the setting.

Headcanon: if a romanced Tav chooses Waterdeep over Baldur's Gate, Gale will spend the next year creating a permanent sanctum that allows Tav to "visit" any part of hometown Baldur's Gate they wish, or even the campsites where they fell in love.