I was wondering if when my subscriptions ends, should I chose not to renew it, do I still get to keep my level 50 GrandMaster and have access to the "free areas" of Wizard City?
The answer you seek is right in the Help & Support Frequently Asked Questions section. Your Wizard is safely stored at Ravenwood School of Magical Arts. Even if you uninstall Wizard101 from your computer, your Wizard, all the belongings and experience earned is still safe.
When you reinstall, it may look like your Wizard is still wearing his/her starter outfit, but they're not, it just takes a little time for your computer to download all images of the clothing and items that you're wearing, so don't panic! You can verify that your Wizard is still wearing their higher level clothing by looking at it in your backpack.
If you are logged off in a subscription area, and your subscription expires, you will be returned to Ravenwood Commons and you can continue to play in the Free to Play areas of Wizard101.
If you want the prize, you have to build the ladder rung by rung. There are no shortcuts.