ListAccounts will return the Gaia ID as well as the email of the account.

chrome/browser/resources/signin_internals/signin_index.html

Index: chrome/browser/resources/signin_internals/signin_index.html
diff --git a/chrome/browser/resources/signin_internals/signin_index.html b/chrome/browser/resources/signin_internals/signin_index.html
index cca049fb7d9fd0ed91a326728ff1f9e03ddf0930..9ab71977acde0b935e39e759bb28fda7d845ef6b 100644
--- a/chrome/browser/resources/signin_internals/signin_index.html
+++ b/chrome/browser/resources/signin_internals/signin_index.html
@@ -58,10 +58,12 @@
       <table class="signin-details">
         <tr class="header">
           <td>Email Address</td>
+          <td>Gaia ID</td>
           <td>Validity</td>
         </tr>
         <tr jsselect="cookie_info">
           <td jscontent="email"></td>
+          <td jscontent="gaia_id"></td>
           <td jscontent="valid"></td>
         </tr>
       </table>