force registered user as seller

I would hook into user_register similarly to how you hooked into gform_post_submission_1 before. For example:

add_action( 'user_register', 'awesome_function_name', 10, 1 );
function awesome_function_name( $user_id ) {
    // Conditional logic for testing user role
    update_user_meta( $user_id, 'dokan_enable_selling', 'yes' );
}

For your conditional logic, if you’re trying to make every role do this except customer I typically use the following function which I don’t remember where I got so I can’t credit them unfortunately.

function check_user_role( $role, $user_id = null ) {
    if( is_numeric( $user_id ) ) {
        $user = get_userdata( $user_id );
    } else {
        $user = wp_get_current_user();
    }

    return in_array( $role, (array)$user->roles );
}

So your action above would be modified to:

add_action( 'user_register', 'awesome_function_name', 10, 1 );
function awesome_function_name( $user_id ) {
    if( !check_user_role( 'customer', $user_id ) ) {
        update_user_meta( $user_id, 'dokan_enable_selling', 'yes' );
    }
}