In the interest of giving credit where credit is due, my avatar is a scan of a hand-drawn caricature by cartoonist Jim Naylor which was done at my company's 20th anniversary dinner.
Is acquiring a card via Masquerade considered "gaining"?
If I Militia my opponent down to just Colonies and Provinces and then force him to pass a Province to another opponent can I react with my Fool's Gold?
No - with Masquerade you pass the card, it is not gained.
Intrigue rules wrote:
"Pass" - when a player passes a card to another player, he places that card face down on the table between himself and the other player. The receiving player then takes the card from the table and puts it in his hand. A passed card is not revealed to the other players. A passed card is not considered to be trashed or discarded by the player passing it and it is not considered to be gained by the player receiving it.