To meet with GDPR, you need to inform to your users about any cookie that your site uses.

Referrals add-on creates a cookie to track when a referral visitor becomes a referral sign up. So, the details of the cookie are:

Cookie name: gamipress_referrals_ref
Expiration time: 1 day

How to change the cookie expiration time?

We provide the filter gamipress_referrals_cookie_lifetime to let you change the cookie lifetime, there is a example to change it for 1 month:

function my_prefix_custom_referrals_cookie_lifetime( $lifetime, $affiliate_id, $referral_id, $affiliate ) {
    return strtotime( '+1 month' );
}
add_filter( 'gamipress_referrals_cookie_lifetime', 'my_prefix_custom_referrals_cookie_lifetime', 10, 4 );