URL replace

Hello,

i have a problem i cant figure out..

I want to create shortcode like [gsm $brand=8810]
that creates a link like this: https://gsm.com/8810

This is what i came up with, but it aint working:

function gsm_url($brand) {
echo "<a href='https://gsm.com/$brand>$brand'</a>";
}

add_action('gsm','gsm_url');
do_action('gsm');

Where do i go wrong?