Gavin DeGraw Sinead Can Tell Fans to F Themselves I'm a 1st Amendment Guy!

EXCLUSIVE
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Gavin DeGraw thinks everyone -- including Sinead O'Connor -- has a right to tell anyone to go f**k themselves ... because this is America dammit!

We ask Gavin about the crap Sinead is both taking and dishing out after she cancelled a few concerts and fans went on an Internet tirade against her.  She blasted back in kind.

We got Gavin after he landed at LAX ... we're guessing he flew American.