If you're experiencing user interface issues with Cirrus Insight (such as the Edit Layout gear icon not appearing), a conflicting Google Lab may be the cause.
To determine if that's the case for you, open the following URL which will load Gmail with all labs disabled: https://mail.google.com/mail/u/0/?labs=0#inbox
If this resolves the issue, then you've determined that Google Labs is causing a conflict. You can close the tab and then access Gmail as you normally do to find the conflicting Lab.
1. Click the Gmail gear icon near the top right of your inbox
2. Select the Settings option from the dropdown menu
3. Select Labs from the top of the Settings page
4. Disable all Labs
5. Select Save Changes button
6. Re-enable each individual Lab one-by-one (saving changes each time) to determine the conflict.
Note: if your Edit Layout gear icon is not appearing in CI, the conflicting Lab is "Right-side chat."