Hi Mohammad,
Thank you.

Looks like

arm_after_add_new_user

Is the correct hook. I will post a solution with it today and then you can check and see if it works.

Regards
Brajesh

Hi Mohammad,
Sorry, I kept you waiting for such a long time.

Please use the updated code

function buddydev_assign_member_type_on_registration( $user_id, $data ) {

	// replace =gender with your field name
	$gender = isset( $data['gender'] ) ? $data['gender'] : '';

	if ( ! empty( $gender ) ) {
		if ( 'male' == $gender ) {
			bp_set_member_type( $user_id, 'member_type_name' ); // please change member type with valid member type.
		} elseif ( 'female' == $gender ) {
			bp_set_member_type( $user_id, 'member_type_name' ); // please change member type with valid member type.
		}
	}
}
add_action( 'arm_after_add_new_user', 'buddydev_assign_member_type_on_registration', 10, 2 );

Please make sure to change the member_type_name with appropriate member types(The unique name/key) as registered.

Also, if it does not work in bp-custom.php, please put it in your theme’s functions.php

Please let me know how it goes.

Regards
Brajesh