If you wish to define a Boolean-valued custom field, then you could replace the constant fgwString by fgwNumeric in the above code examples. However, since every variable in VBScript is a Variant, you could assign a Boolean value to a string-typed field, and the value will be stored as a valid string representation.
In some cases you may need to use explicit type coercion when writing and reading a Boolean value (but this is not usually necessary), as in the following:
oCheckBox.Checked = CBool(oField.Value)
You can review the range of available constants in the Formativ Language Guide. To see these, go to the Contents tab and select Reference | Constants | eFieldType.
I hope this information helps you.