Update your Bookmarks to https://sites.google.com/view/gam--commands/home
If you do not need to check sub-OUs, then replace ou_and_children with ou.
gam print chromeapps ou_and_children </Path/To/OU> todrive
gam delete chromepolicy chrome.users.NewTabPageLocation ou </Path/To/OU>
gam print chromepolicies ou </Path/To/OU> todrive
gam print chromepolicies ou </Path/To/OU> filter chrome.users.ManagedBookmarksSetting todrive
As a json file
gam print chromepolicies ou </Path/To/OU> filter chrome.users.ManagedBookmarksSetting formatjson todrive
gam update chromepolicy chrome.users.NewTabPageLocation newtabpagelocation "<New Tab URL>" ou </Path/To/OU>
Create the JSON file, of the bookmarks, by first using the print ... formatjson command. Copy the JSON info into a TXT file and save as .json. Edit this file and use as needed.
gam update chromepolicy chrome.users.ManagedBookmarksSetting json file <File Name>.json orgunit </Path/To/OU>
This example creates 3 subfolders (Systems, Forms & Info) under the Bookmarks folder and lists the bookmarks inside these sub-folders as needed.
Use this Google Sheet to create the JSON 'code' for you bookmark file.
Insert rows where needed and enter the Name & URL into columns A & B.
Copy cells in columns C to H as needed to enable the autofill.
Note that each line of 'code' in column H contains a comma at the end.
Amend this JSON file as needed.
Open the JSON file in a text editor.
Copy the Column H (excluding heading/row 1) from the Google Sheet into each section of the JSON file.
Note that the bookmarks are listed above the folder name in the JSON file. Just replace the lines with "name" & "url", see image below.
Delete the last comma of the section in the JSON file.
Save the JSON file.
gam update chromepolicy chrome.users.ManagedBookmarksSetting json file <JSON file name>.json orgunit </Path/To/OU>