Are you referring to the left navigation menu below? That’s not possible. 

 

You can add items to the navigation menu on the C360: 

 

Thanks for the reply!

I was referring to the navbar:

Not the sidebar.

I saw in Salesforce that it is editable as an admin.

In GS I am also super admin, but I couldn’t find any documentation that covers this.

And I noticed that the items in the navbar are different than what’s available in the sidebar (for the GS landing page). The ones in green are extra.

And I want to add Administration as a link at the top, for instance. 

Or Reports… 
Since I don’t want to go to the Gainsight tab each time I want to access these functions.

Update:
It is doable and to quote the Mandalorian, “this is the way” ( 1:30 ): 
 

I spoke with GS Support and if the pencil is not available, as with my instance, we must speak with a Salesforce admin to upgrade the instance to the lightning view.

I just had this, instead of the editing pencil:

Bare in mind, I discovered there can be 3 different views of Gainsight, accessible via lightning experience view or app switch from the upper right or Gainsight tab click without app switch:

1 Gainsight app switch from upper right:

2 (lightning view):

3.Sales app, but with Gainsight tab clicked refreshes the page and shows configured Gainsight tabs in classic view:

via the Customize screen, accessible from here: https://share.getcloudapp.com/BluxXYP9

Be sure to select Gainsight app when adding items

My quest was to see how to add tabs to the lightning view of GS , but I must speak with an admin to do the upgrade.

Correct! It’s doable in Salesforce. Not in Gainsight. 

But wouldn’t that Info icon change to a Pencil icon if the SF admin would upgrade GS to Lightning as stated in the information pop-up ?
 

That’s what support said; that after we talk with the SF admin, we can add tabs to Gainsight (in Lightning exp)

And the screenshot I appended in my earlier post, shows the Classic view of Gainsight showing the tabs that I added via the “+” icon from SF, but available in the Gainsight app - as shown in the upper right.