ButtonGuiPackets.java

//////////////////////////////
//// MOD FILE
//////////////////////////////

public static SimpleNetworkWrapper packets;

@EventHandler
public void postInit(FMLPostInitializationEvent event) {
        packets = NetworkRegistry.INSTANCE.newSimpleChannel("SomeName");
        packets.registerMessage(GuiRequestPacket.class, GuiRequestPacket.class, 0, Side.SERVER); //Packed id, 0-31
        }


//////////////////////////////
//// GUI FILE
//////////////////////////////

@Override
protected void actionPerformed(GuiButton button) {
        // you need to store entity instance when opening gui, trader in my case.
        if(button == yourbutton) Core.packets.sendToServer(new GuiRequestPacket(3, this.trader.getEntityId())); // 3 is your gui id (from your code)
        }