To import a SmartNode T1 Gateway xml template into 3CX, you will first need to download the template from here.  Once you download the file and extract the "Patton-T1.gw.xml" from the zip file, you will need to place it in the following location.

C:\ProgramData\3CX\Instance1\Data\Http\Templates\gateway

Note:  You may need to go into your PC's folder options, click the "View" tab and make sure that under "Hidden files and folders" that "Show hidden files, folders, and drives" is selected.

Once there, you can either delete the existing "Patton-T1.gw.xml" file and paste in the new file or rename it.  Once this is done, you will walk through the same steps to create the PSTN gateway and it will now configure the SmartNode as expected.