Code:
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import std.bigint;
import std.numeric;
import std.stdio;
void main() {
	const a = BigInt("123143238472389492934020");
	const b = BigInt("902380489324729338420924");
	writeln(gcd(a, b));
}
----------

Compiler output:
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/usr/src/d/phobos/std/numeric.d(3044): Error: template std.bigint.BigInt.opAssign cannot deduce function from argument types !()(BigInt) const, candidates are:
/usr/src/d/phobos/std/bigint.d(178):        opAssign(T)(T x)
/usr/src/d/phobos/std/bigint.d(194):        opAssign(T : BigInt)(T x)
/usr/src/d/phobos/std/numeric.d(3045): Error: template std.bigint.BigInt.opAssign cannot deduce function from argument types !()(const(BigInt)) const, candidates are:
/usr/src/d/phobos/std/bigint.d(178):        opAssign(T)(T x)
/usr/src/d/phobos/std/bigint.d(194):        opAssign(T : BigInt)(T x)
test.d(7): Error: template instance std.numeric.gcd!(const(BigInt)) error instantiating
----------

There's no reason why const BigInt shouldn't work with gcd, since gcd doesn't (shouldn't!) modify its arguments, and the following does work:

----------
import std.bigint;
//import std.numeric; // NG
import std.stdio;

// Copy-n-pasted code from std.numeric.gcd
BigInt gcd(in BigInt a, in BigInt b) {
        BigInt ua = a;
        BigInt ub = b;
        while (ub)
        {
            auto t = ub;
            ub = ua % ub;
            ua = t;
        }
        return ua;
}

void main() {
        const a = BigInt("123143238472389492934020");
        const b = BigInt("902380489324729338420924");
        writeln(gcd(a, b));
}
----------

Program output: 12