To get a data in nebula grid it is required to call a method getText(int column). Since method has a parameter it is not accessible via Advanced properties.
The fix might be to extend a get-property to understand method calls with parameters, so the script will look like this:
get-canvas | get-property "getItems.getText(1)" | equals "ColumnValue" | verify-true
Or, which I guess is a more preferable way, add a general ECL command like call-method, so the following script might work:
get-canvas | get-property "getItems" -raw | call-method getText 1 | eq "ColumnValue" | assert-true "Unexpected column value"