Deleting the Gmail app wouldn't give you extra space, as it came pre-installed as system app. Those "system apps" live in a different partition (
/system, which is why they are usually called "system apps"), which is mounted read-only – and not available to the user to be written.
However, Google Apps (including Gmail) are regularly updated via Google Play. Those updates are placed in user-space (inside
/data), and thus consume space you could use otherwise. To claim back that space from system apps you don't need:
- From the home screen, go to Settings › Apps
- Select the "All" tab
- Scroll to the app in question (here: Gmail), tap its entry
- Now you see some details on that app, including some buttons you can press. Press:
- Force Stop to "shut down" the app
- Clear Cache to make it release its "temporary files"
- Delete data to get rid of its configuration and stored user data (optional; if you think you might need this app later, skip this step)
- Uninstall updates to get rid of the app-updates installed in user-space
- Now that button gets renamed to Disable, hit it again (it should be renamed now a second time, and read Enable – so in case you need it at a later time, you can do so. Note it then would draw-in the latest updates again)
Now the app should no longer be visible in the app drawer, and also its shortcuts etc. on your home screen(s) should be gone. Storage claimed by the app's updates, caches, and (optionally) data is freed, and available to you to be used for other purposes.