Two simple things you can do (also in Perl 5):

You can get rid of the temporary $t by using list assignment:

    while $b != 0 {
        ($b, $a) = ($a % $b, $b);
    }
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Or you can resort to recursion, ignoring your boundaries to the golf court:

sub gcd ($a, $b) {
    return $a if !$b;
    gcd($b, $a % $b);
}

say gcd(24, 42);
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For none of these I can see something where Perl 6 really plays its strengths.

So maybe something completely else? This might work, but since nobody implements infix:<...> yet, I couldn't test it:

sub gcd ($a, $b) {
    ($a, $b ... { $^x % $^y || () })[*-1]
}
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For example for gcd(42, 24) it would produce the list 42, 24, 18, 6. During the next call to the block it the $^x % $^y returns 0, and the block the empty list, ending the creation of the list. The last element is the gcd.