Maybe I have a better understanding of what's overall going on now. Let's discuss a few topics.
A Gratuitous ARP really isn't a request for information - it's a "Oooh, Oooh, Look at me, I'm here on the network!" kind of message. Most often Gratuitous ARP messages are sent out from systems so that they're added to ARP tables automatically, can be configured to a switch quicker, and other benefits - a Gratuitous ARP message is actually a "forced ARP Reply" even though there was never an ARP Request (it rarely ever solicits a reply from anyone else on the system).
If you're trying to listen to data on a TCP port, a system would have to specifically open a socket to your specific IP address (or you'd have to connect to the port on the remote system) - TCP is targeted, meaning it's a 1-to-1 protocol. Unless your Siemens PLC knows about your presence and directly sends you information, you won't receive anything from it. You cannot "listen in" on TCP conversations happening (without specific hardware, hardware configurations, or packet mirroring/monitoring functionality on the switches being used). You likely will never see this traffic in WireShark unless the traffic is specifically being sent to YOU, or you've got one of the aforementioned hardware/software configurations that allows you to see it. In short, it's not likely you're ever going to be able to just "eaves drop" on a TCP conversation. With TCP, your best bet is to initiate the conversation yourself (though you have to know how to do that with the Siemens PLC).
If you're trying to listen to data on a UDP port, you've got a much better chance of hearing what someone is saying. UDP Broadcast or UDP Multicast (where you're registered to the same multicast address) allows you to "listen in" on what's going on, but only if the system is "broadcasting to the world" the information. You'd see this traffic loud and clear in Wireshark, though. If you aren't seeing this traffic clear-as-day in Wireshark, then likely it isn't being transmitted. And you cannot listen in on a conversation (even a one-way conversation) that isn't happening.
So, to wrap a bow on this, everything is likely working properly. Your Siemens PLC is just not sending out any data that your LabVIEW app can listen to (LabVIEW isn't a mind reader, after all). You likely need help from a Siemens PLC forum, not a LabVIEW forum.
EDIT: Page 32 of the Siemen's LOGO! user manual says that you can only create a network topology with a Siemen's LOGO! through the use of their LOGO!Soft Comfort V7.0 software. This usually means the network communication is proprietary, and they're not going to tell you how to communicate with the device via generic TCP/IP. I guess that's what happens when you use a PLC. 
-Danny
