The xml tag names are defined in the file components/com_googlebasexml/models/googlebasexml.php.
Look for the function getTags() - if you are using a version of the component prior to 2.0.0 then it will be around line 870, if you are using 2.0.0 then it is around line 1100.
The function is self-explanatory, it simply defines the tag names, eg
$tags->description = 'description';
$tags->id = 'g:id';
$tags->link = 'link';
$tags->price = 'g:price';
$tags->sale_price = 'g:sale_price';
$tags->sale_price_effective = 'g:sale_price_effective_date';
You can change the values to whatever you want.
