Hi
It depends of the customization level that you've done.
What i recommend is that you backup first your joomla!1.7 website (use for example Akeeba) and them install it locally.
Them upgrade your joomla, remove old and install new J!2.5 template version.
Also upgrade all GK extensions used on this template.
Now that you have all updated, you can check the settings and changes used on the template from on your local installation.
I think this is the more secure way.
Strong Recommendation:If you make css customization on your template, please enable override.css file and place there all your work. This way there's no problem for future updates.
Other suggestion:If you need to compare your old files with new ones, you can use for example:
DiffMerge SoftwareExcellent tool and free.
Cheers