PLC as gateway with TCP/IP communication stack?
MODBUS TCP, TCP, and IP are all different layers that stack up to form a complete communications channel.  

IP is the network layer, and handles how packets of data get moved from node to node across a network (or between networks).  In the OSI model, this is Layer 3.  

TCP is a transport layer (Layer 4) that operates 'on top of' the IP layer, and provides connection-based logical link between devices on the network.  

MODBUS TCP is a protocol that is transported via TCP, and is essentially a networked version of the pre-existing MODBUS protocol that's commonly used over serial links in industry.

So in order to use MODBUS TCP, you need to have the underlying layers in place first.  You need two devices that support TCP connections, and TCP needs IP, and IP in turn needs a data link layer and physical medium to move across (usually Ethernet, but could also be fiber or radio).

In addition to MODBUS, there are other transport protocols that work over TCP--you are reading this text thanks to HTTP.  Likewise, you can use IP without TCP, for instance there are a number of protocols that use UDP, which is connectionless and so simpler to implement, and facilitates multicasting.

Before you can determine the best solution for your application, you'll need to better define your needs.  MODBUS is based on reading and writing to logical "registers" and "coils" (essentially bits).  If that functionality maps well to your application, then off-the-shelf devices that implement MODBUS TCP could be a good fit.  If you need more flexibility in the structure of the data or the nature of your transactions, then you'll need to find or create a different protocol.  

If you want an embedded device, there are plenty of microcontroller platforms and dev boards that provide Ethernet connectivity with good support for bringing up a TCP stack in software.  But a good place to start may be on a PC, where you can write more sophisticated software more quickly to model your data flow and prototype some potential solutions.