If you want the bot to read the .gz file, you put the .gz name in the index. That is:
<sitemap>
<loc>http://www.mysite.com/SiteMap1.xml.gz</loc>
<lastmod>2013-08-20</lastmod>
</sitemap>
<sitemap>
<loc>http://www.mysite.com/SiteMap2.xml.gz</loc>
<lastmod>2013-08-20</lastmod>
</sitemap>
See Using Sitemap Index Files.
The same thing goes for your robots.txt file: put the name of the gzipped file.