To disable the module you must login to your server's terminal, go to Magento install directory and perform:

php bin/magento module:disable Unirgy_Giftcert

If full removal is needed, then all extension's files should be removed. Files are located at:

app/code/Unirgy/Giftcert/

If you want to remove all database traces of the module you should:

DROP TABLE `ugiftcert_cert`;
DROP TABLE `ugiftcert_history`;
DELETE FROM `eav_attribute` WHERE `attribute_code` LIKE '%giftcert%';
DELETE FROM `catalog_product_entity` WHERE `type_id`='ugiftcert';

Removing databse entries is not required except of

DELETE FROM `eav_attribute` WHERE `attribute_code` LIKE '%giftcert%';

which will prevent any PHP errors due to missing classes for GC attributes. If you want you can scan sales_flat tables for any %giftcert% columns and remove them.