GMP not available in XAMPP and WAMP?

Hi there,
I was wondering whether it's something wrong on my side, or GMP is not enabled by default on these systems. Here's my code:
[code]
<?php
$number = gmp_fact(1000);
echo gmp_strval($number);
?>
[/code]
I get:
Fatal error: Call to undefined function gmp_fact() in C:\xampp\htdocs\test.php on line 2
If this means that it is in fact disabled, how may I enable it?
Thanks!
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