To disable the module it is enough to edit app/etc/modules/Unirgy_Giftcetrt.xml and make sure <activ>true</active> is changed to <active>false</active>. Disabling from Admin System>Configuration>Advanced is not going to work.
If full removal is needed, then all extension's files should be removed.
These are:
app/etc/modules/Unirgy_Giftcetrt.xml
app/code/community/Unirgy/Giftcert/
app/design/adminhtml/default/default/layout/ugiftcert.xml
app/design/adminhtml/default/default/template/ugiftcert/
app/design/frontend/base/default/layout/ugiftcert.xml
app/design/frontend/base/default/template/unirgy/giftcert/
js/colorpicker/
app/locale/en_US/Unirgy_Giftcert.csv
app/locale/de_DE/Unirgy_Giftcert.csv
app/locale/fr_FR/Unirgy_Giftcert.csv
app/locale/it_IT/Unirgy_Giftcert.csv
app/locale/ja_JP/Unirgy_Giftcert.csv
app/locale/nl_NL/Unirgy_Giftcert.csv
app/locale/en_US/template/email/ugiftcert_self.html
app/locale/en_US/template/email/ugiftcert_recipient.html
app/locale/de_DE/template/email/ugiftcert_self.html
app/locale/de_DE/template/email/ugiftcert_recipient.html
app/locale/fr_FR/template/email/ugiftcert_self.html
app/locale/fr_FR/template/email/ugiftcert_recipient.html
app/locale/ja_JP/template/email/ugiftcert_self.html
app/locale/ja_JP/template/email/ugiftcert_recipient.html
app/locale/pt_PT/template/email/ugiftcert_self.html
app/locale/pt_PT/template/email/ugiftcert_recipient.html
skin/adminhtml/default/default/ugiftcert.css
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.