If after installing Gift certificate module, you are missing gift certificate form on product page, there is grat chance that your theme has been modified in such a way that it is missing some important calls.
File to look at is
app/design/frontend/<packacge>/<theme>/template/catalog/product/view.phtml, in that template you should have a call like:
<?php echo $this->getChildHtml('product_type_data');?>
That call is responsible for rendering product type specific details. That call MUST be placed within add to cart form on product page, otherwise Giftcert details will not be submitted and you will still get error messages.
If you don't want to have this call for all product types, you can surround it in conditional block like:
<?php if($_product->getTypeId() == 'ugiftcert'): ?> <?php echo $this->getChildHtml('product_type_data');?> <?php endif;?>
If using Magento versions less than 1.4, the solution above will not be enough. Reason is that current version of Giftcert has its template files in base/default theme, which is available since 1.4. To enable certificate to function properly, you will need to copy frontend/base/default/layout/giftcet.xml to your theme layout folder and frontend/base/default/template/unirgy folder to your theme template folder.
If you have previously used older version of the plugin like 0.9.x or have made template/layout modifications while using 1.x version of the plugin, then templates and layout of current branch 2.x.x is not compatible with older versions.
Therefore for old version of the plugin - check all themes BESIDES base/default and make sure to remove any unirgy/giftcert template folder and ugiftcert.xml layout file. The only templates should be in frontend/base/default/template/unirgy/giftcert and only layout in frontend/base/default/layout/ugiftcert.xml
Remove any other copies of these files and then test adding Gift Certificate product to cart. If that works, then you can look into customization of the templates if you need that.