Solution
There is not an NI out-of-the-box solution to Read and Write text files in G Web Development Software, if you would like to complete these tasks you can either call a JavaScript or a LabVIEW Web Service.
The
File for WebVI library has been also developed by one community member, it integrates JavaScript functions to complete tasks related to File I/O. This is not an NI Official Supported Library but can be useful for development if needed.