Definitely gzip sitemaps because that lowers downloading time, thereby making the server faster more often. Let google do the decompression at their end for you. If timing is an issue, what you want to do is click the gear icon and visit "site settings" in webmaster tools for all websites you have that the sitemap is taking forever to process, then for the crawl rate, choose the maximum number available. Usually google will allow you to set it so google crawls no more than two pages a second, but if google likes your content, then in a couple of days or so, they will let you increase the crawl rate beyond two pages.
Then mathematically speaking, provided google never does anything else but checks your pages (which I doubt), The time you need for google to scan every file in your sitemap is:
500,000 pages divided by two pages a second
= 250,000 seconds
or 4,166.6 minutes
or 69.4 hours
or roughly three days.
What I suggest since you're on the right track so far is to either wait at least 30-60 days for google to finish scanning every page in your sitemap or keep checking the site settings and pray you can increase the crawl rate. I had it at one point where I could have google crawl my site containing about 1 million links at up to a rate of 10 requests per second.
Just make sure the content of every page in your site map is of high quality as defined in google's webmaster guidelines or your SEO will hurt.