It has been a while since I used the FMCW block in the Radar toolbox, but from what I can remember, the three samples sections are defining the percentage of the signal you want distributed in each of those sections. For example, if you want 50% of your signal to be CW, and 50% to be up-chirp, put an equal number of samples in the Samples CW and Samples up-chirp sections. The number of samples combined with your sample rate and frequency sweep will determine the slope of the fm chirp.
There is also a VCO block I'm pretty sure that you can use for more customizable fmcw radar.
For your second question, why do you want to add a dead time? My guess would be because you are trying to do pulse doppler of some sort, but if not please explain because I would definitely be interested in knowing what you are attempting to do! If you are trying to do a pulse doppler radar with a linear fm pulse compression, unfortunately, there isn't much functionality within GNU Radio for that kind of thing, and SDRs aren't great for pulse doppler right now. In a few years' time they will, but most affordable SDRs just don't have the hardware capabilities necessary. FMCW is the way to go for SDRs because of low peak power needed and because timing is not as necessary for range disambiguation, which is nice because of the low sample rates of most SDRs.
I am not sure you can implement a dead-time within the FMCW signal generator block, and am not sure that gr-radar has that capability. However, if you still want to implement a dead-time, you could write a custom block (GNU Radio has extensive tutorials on how to do this, and only requires basic knowledge of yaml and c++). I would do something like, generate a sine wave, and while the sine wave is above a certain threshold, send the fmcw signal through, else, send a constant 0 through. You would have to play around with the threshold to get the proper duty cycle and play around with the frequency of the sine wave to make sure you are staying in sync with the signal generator, but that is one idea on how I would do it off the top of my head.
Can you give me more information on the project and/or what your goals are? It would help me give more specific advice as opposed to shooting blindly.