Instead of DTSGlobalVarialbes("gVar"), put DTSGlobalVarialbes("gVar").Value

CREATE FUNCTION [dbo].[fn_FileExists] (@FileName nvarchar(255))
RETURNS bit AS
BEGIN
DECLARE @FS int
DECLARE @FileID int
DECLARE @OLEResult int
declare @Exists bit

EXECUTE @OLEResult = sp_OACreate 'Scripting.FileSystemObject', @FS OUT
IF @OLEResult <> 0
BEGIN
RETURN 0
END

EXECUTE @OLEResult = sp_OAMethod @FS, 'FileExists', @Exists OUT, @FileName

IF @OLEResult <> 0
BEGIN
return 0
END
EXECUTE @OLEResult = sp_OADestroy @FS

return @Exists

END

Dim fso, file1
Dim pkg
SET pkg = DTSGlobalVariables.Parent

Set fso = CreateObject("Scripting.FileSystemObject")

IF (fso.FileExists("c:\abc.txt")) THEN
Msgbox "File exist. Continue"
pkg.Steps("DTSStep_DTSActiveScriptTask_2").DisableStep=False

Set fso= Nothing

ELSE
Msgbox "Stop"
pkg.Steps("DTSStep_DTSActiveScriptTask_2").DisableStep=True
END IF
set pkg=nothing
Main = DTSTaskExecResult_Success