By default, Web Designer Galleries settings are missing on Sharepoint Online Site Settings Page. The reason behind this situation is because this options requires scripting which causes security issues. Good news is; you can enable it!

How to Enable?

Log on to https://portal.office.com and click Admin.

On the left panel, under Admin Centers click Sharepoint. This will redirect to https://<tenant>-admin.sharepoint.com.

Once the Sharepoint Admin Center opens, select Settings which is on the left menu and change the Custom Script settings as shown below.

Your Web Designer Galleries tab in Site Settings page will appear in 24 hours. Keep checking.