Is this waterfall characteristic of QAM or OFDM (or something other) modulation?
The sharp rectangular shape of the spectrum, with the immensely flat top definitely says "OFDM, with suitable whitening/PAPR reduction"; what constellation is used on the individual carriers is a bit hard to tell.

Can you, with a no-samples-left-out spectrogram (e.g. gr-fosphor sink, or by playing back your recorded file slower and using a suitably high update rate and suitable low number of FFT points in your QT waterfall sink, determine how long the OFDM "burst" is?

Take a bunch of samples from the end, and a bunch of samples from the start (like, one fourth of the burst duration), and calculate a cross-correlation. The maximum correlation should give you the time when what is used as the cyclic prefix at the end of the burst starts; that way, you'll be able to make an educated guess at the symbol number that makes up the CP, and by that the OFDM symbol length/IFFT size. Take frame, and try to manually resample it, so that both CP starts are exactly that (guessed) IFFT size apart. FFT the hell out of the frame; because your resampling had the purpose of time-stretching your signal so that the length of it actually (hopefully) matches the transmitter's sampling, you'd have a timing-synced demodulator. 

Look at the spectrum and the constellation plot of the resulting signal; maybe you can already guess the constellation used to modulate the OFDM subcarriers from that.