Alert email - converting custom property value to GB

I have created a custom property on volumes called DiskSpaceAlert  -- this field will allow us to customize the level of disk space at which alerts are generated. So I have the trigger condition setup properly - but I'm trying to convert the value we've entered in DiskSpaceAlert back to gb when the alert email is generated. So, I've added this statement into the alert email:

${Caption} is below the value (${DiskSpaceAlert}/1073741824, 0) gb

What I'm expecting is this:

C:\ Label: diskdisk is below the value 5 gb

But what I'm actually getting is this:

C:\ Label: diskdisk is below the value (${DiskSpaceAlert}/1073741824, 0) gb

I've reviewed similar information on the forums, but can't find the answer to what I'm doing wrong here...?

Thanks.