Kim is right in advising you to be aware of possible regulatory issues surrounding the name of these things.
Text phrases such as these are generally (almost always) controlled by LANGUAGE DEFINES, and are located in the php files in the languages folders.
It is perfectly safe to change these text strings to whatever you want, as long as you do not touch the KEYS or IDENTIFIERS.
For example, in the following DEFINE, I do not touch the RED part, but can put whatever text I like in the GREEN part.
define('HEADER_SEARCH_DEFAULT_TEXT', 'Enter search keywords here');
Now, on the matter of Gift Certificates... remember, these are VIRTUAL (electronic) certificates. They are never "printed out" and sent to people. So they can never exist as real "cards".
They are simply a string of code, that, when redeemed online, put a CREDIT VALUE into the shoppers online account.
This bit of code (like cash) is negotiable, meaning that you can e-mail the code to anyone you like, and if they enter it into the redemption field in your webshop, they get the cash credit.