Practical Gardner timing error implementation I'm trying to demodulate a QPSK communication.
For the moment I do:


  
  Signal detection by computing the ratio between the power of the carrier wave compared to the signal power.
  Initial clock synchronization with a preamble.
  Initial phase recovery with the same preamble.
  


This works great with my signal but now I want to go further and take into consideration a clock frequency offset betweem the TX Clock and the RX Clock.

I made some research and a good way to do it seem to be Gardner Timing Error Detector which should be able to compensate for my clock offset.

To implement it I do:
$$\epsilon_k = Real[(signal(t_k)-signal(t_{k-1}))*conj(signal(t_{k-1/2})]$$
$$t_k = t_{k-1}+T+\gamma*\epsilon_k$$

My problem is that I don't know how to chose the proportional parameter.
$$\gamma$$

I've sometime seen it with something called the normalized narrowband noise BlT but I have no idea what it is. If you could point me in the right direction any help would be very appreciated!
If you have access to Mengali's book 'Synchronization Techniques for Digital Receivers', you will find the details how to choose it for a tracking loop, be it for frequency, phase or timing. Otherwise, just read about a standard discrete-time phase locked loop and see how the choice of paramters affect the loop bandwidth, which is what you are after.