GPG Encryption in .NET
The following VB.NET code may be used in conjunction with GPG4Win to GPG encrypt a file using a .NET web application prior to sending to a specified recipient. This approach calls the GPG4Win command line utility to import a key and encrypt a file via the System.Diagnostics namespace.
This code works with GPG4Win version 3.1.10.

First, download and install GPG4Win on the server where this code will run
You may need to add an Imports System.Diagnostics and/or System.IO statement(s) at top of page.
The directory where you write the file will have to have the correct permissions based on your application pool settings.
Your application pool user must have write permission on the target directory.
Be sure your source directory is private/not exposed to the web.
This produces an encrypted copy of the original file with .gpg appended.
Be sure to delete the unencrypted version (if needed) once the encryption is done.

                Dim strFileFolder as String = "C:\Files\" 'The directory of the file to be encrypted - be sure this is a private directory
                Dim strFileName as String = "MySpreadSheet.csv" 'The name of the file to be encrypted
                Dim strKeyFile as String = "C:\Keys\KEYFILE.asc" 'The complete path to the public key file provided by the recipient.
                Dim strRecipient as String = "RecipientName" 'should match the name or email in the GPG4Win Keychain entry

                Dim strFilePath as String = strFileFolder & strFileName '

                Dim gpgImportError As String = ""
                Dim gpgEncryptError As String = ""

                'call the gpg4win process
                Dim gpg = New Diagnostics.Process()
                gpg.StartInfo.FileName = "gpg"
                gpg.StartInfo.UseShellExecute = False
                gpg.StartInfo.RedirectStandardError = True

                'set import key location args
                gpg.StartInfo.Arguments = " --import " & strKeyFile

                'import the key 
                gpg.Start()
                gpgImportError = gpg.StandardError.ReadToEnd()
                gpg.WaitForExit()

                'show confirmation or output if there is an error
                If gpgImportError <> "" Then
                    Response.Write("GPG IMPORT ERROR - " & gpgImportError & "|")
                Else
                    Response.Write("OK - KEY IMPORTED|")
                End If

                'set encryption args --trust-model always ensure no user feedback/prompt. 
                'Use this setting with caution, and only if you are certain of the integrity of the key
                'since no output is specified, it will encrypt into the same directory and append a .gpg

                Dim strArgs = "--encrypt --trust-model always --recipient " & strRecipient & " " & strFilePath
                'Encrypt the file.
                gpg.StartInfo.Arguments = strArgs

                gpg.Start()
                gpgEncryptError = gpg.StandardError.ReadToEnd()
                gpg.WaitForExit()


                'output the args - to call from a command line, call gpg with these args
                Response.Write("gpg " & strArgs & "|")
                If gpgEncryptError <> "" Then
                    Response.Write("GPG ENCRYPT ERROR - " & gpgEncryptError & "|")
                Else
                    Response.Write("OK - ENCRYPTED|")
                End If
                Response.Write("ENCRYPT COMPLETED AT: " & DateTime.Now() & "|")
                gpg = Nothing

                'uncomment below to delete the unencrypted source file
                'IO.File.Delete(strFilePath)
                'Response.Write("SOURCE FILE DELETED|")
                'You should be left with just the encrypted gpg file like "C:\Files\MySpreadSheet.csv.gpg"