LPG -- aka Joe Ruback -- is maybe the most famous Giants fan on the planet ... he's been to EVERY G-men game for nearly TWO DECADES, always showing up with NY license plates around his neck.

But, with New Jersey passing a rule this month that says no fans can come to MetLife Stadium this season ... Ruback's streak is now over -- and he tells us he's feeling pretty blue about it.

"We can sit here and talk about how devastated I am, and then you get the people that are like, 'Oh, you don't care about people's health then!'" the 51-year-old fanatic tells us.

"But, no, we're just talking football. Let's just talk football, OK? I'm devastated, yo! I mean, I'm devastated!"

Ruback, though, says he's still planning to make the most of the situation ... promising to throw some epic watch parties to make up for not being able to cheer on Saquon Barkley in person.

"If you think I'm going to sit here in my basement, you got another thing coming," Ruback says.

"I'm going to get a caterer, I'm going to get a DJ, I'm going to drink, eat and I'm going to watch on a big-ass screen wherever it is."

Fortunately for LPG ... his ties to the team are still strong -- he tells us he was just in contact with star receiver Golden Tate about the upcoming season this week.