There's been some subtle miscues in this thread. Let's start over from the fundamentals.
First, think about what the flowgraph is. You must understand the transmit and receive chains to do meaningful work with GNU Radio flowgraphs.
This is a rather old example of sending packetized data that doesn't use error correction. There is a function that checks the CRC of packets. If a single bit is wrong the packet fails. (As a side note building packetized/bursty modems in the way these flowgraphs work is NOT recommended).
Given this information, for the same SNR and channel what would you expect to happen to bit error rates (and therefor packet error rates) as you change modulations from GMSK, BPSK, etc to QAM?
Finally, as I mentioned these flowgraphs are sort of deprecated. Maybe you can tell us what you're really going for and someone can suggest a better tool.
Cheers,
Nathan