Resolution:
For EDI-Gateway to work in passthrough mode while receiving inbound files, you would have to configure the following steps:

1. Select EDI-Gateway in Protocol Editor of Configuration GUI
2. Create a "new transmission" called TEXT after selecting transaction type from the drop down list to be "Text Document"
3. Enable File poller under Server Settings-&gtPartner to Host Transports
4. Create a new trading partner and configure FTP-Get transport within the trading partner to pick up a particular text file after specifiy the FTP location, input mask etc.
5. Once the engine is started up the EDI FTP-Get poller will pick up the file from the FTP-Get location and pass it to the private process without performing any validation, the message published to private process will be like

2004-02-10 16:17:30 (2004-02-11 00:17:30.062000000Z): subject=AX.BC.&ltinstallation name>.EDI-Gateway.RESPONDER.REQUEST, message={^pfmt^=10 ^ver^=30 ^encoding^=2 ^type^=1 ^prefixList^={default="/tibco/public/scalar/ae" 1="/tibco/public/class/ae/EDI"} ^data^={^class^="ResponderRequestGateway" ^idx^=1 sourceTP="sourceTP" destinationTP="destinationTP" operationID="EDI/TEXT" requestFile="C:\temp\MBF\log\In\StVincents\2004-02-10\BC350EDI261-P4-BVISWANA-hub-soruceTP-2004-02-10--16-h-17-m-29-s-150-ms.dat" deleteRequestFile=true operationType="EDI/Inbound" standardID="EDI-Gateway" receivedTime=2004-02-11 00:17:28.718000000Z} ^tracking^={^id^="f1lSohkAcSxoXUE3iczzw4SEzzw" ^1^[1]="&ltinstallation name>-EDI/Inbound-Job-10517"}}

and the text file will get transferred based on the outbound transports configured.

How to enable EDI-Gateway to act in passthrough mode for transferring text files?.