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.