Yes, you can call that function anywhere!
If you are trying to show a gallery field then here is the code:
<?php echo nggcf_get_gallery_field(1, "Field Name"); /* where 1 is the gallery id */ ?>
If you are inside the foreach in the album template, you can do this:
<?php echo nggcf_get_gallery_field($gallery->gid, "Field Name"); /* This will show the "Field Name" custom field for all galleries in the template */ ?>
If you are trying to show an image custom field, then you would use <?php echo nggcf_get_field(123, "Field Name"); /* where 123 is the ID of the image you want the custom field to show for */ ?>
An explanation for people comfortable with a bit of PHP:
The function nggcf_get_field() takes 2 arguments. The first (int) is the image id from NextGEN, the second (string) is the custom field name. It will return the value for that custom field for the specified image.
The function nggcf_get_gallery_field() is the same, except its first argument is the gallery id.
You should be able to call these functions pretty much anywhere in the wordpress templates
PS code examples untested :)