GNU radio, FSK channel estimator
In FSK, you don't care about the frequency-varying phase properties of your channel – they simply don't matter to your signal, so a (relative) amplitude estimate for all mark frequencies would technically suffice.

Most FSK receivers don't even do that – it's only helpful to know that "mark frequency A gets through stronger than frequency B" if you use that to adjust your decision boundaries (e.g. for an MAP decision, you'd need that knowledge).

I don't have access to any DASH7 phy layer documentation, but if it's really derived from RFID tag technology, then you can be 100% certain the thing works without channel state information. Complexity wouldn't allow that. That doesn't mean you can't make it work better at further distances – but it does imply that if you're well within the specified ranges and environments, getting a channel estimate will not help you significantly much. 
Wikipedia says "range up to 2 km, less than 200 kHz bandwidth", and that basically means your second observable multipath component would have to have a 1.5 km longer way to travel than the first one you receive – that is physically practically irrelevant, as that delayed version of the signal will be so much weaker than the quicker path that it can't lead to significant frequency-selectivity.

So the answer to your question is "you don't do that at all".

The rest of this answer, hence, is of merely academic interest:

Now, if your channel is actually strongly frequency-selective, so that knowing the relative amplitude becomes desirable, you can also be sure that the transition between different frequencies (which is Gauss shaped, transmitter-side) isn't nice anymore – and that's bad for your Gauss-expecting receiver.

Now, the only thing that could be FSK-specifically done here is some form of nonlinear emphasis on some values – that really doesn't sound very tempting, because estimating the necessary distortion to counteract the nonlinear memory-affected effect of a frequency selective channel on the FM demodulation of a frequency transition sounds pretty hard. And whenever possible, we try to compensate systems where they are linear – that gives us at least some guarantees about invertibility/well-posedness of the inverse problem.

So, don't try to deal with your FSK in any special way. Instead, just do what works for any transmission type:


transmit something you know – for example, a preamble, or a known data packet – that is as white as you get with the given scheme (in this case: transmit pseudorandom white data through your GFSK transmitter)
Correlate with that at the receiver
Since that correlation will be mathematically equivalent to (noisy) convolution of your transmit signal's autocorrelation function and the channel impulse response: It's your channel estimate. You don't need it any better than that, because it excites exactly the frequencies present in your signal.


The first step – transmission of something you know – is in many systems done via a decision feedback equalizer, where "something you know" is what you received and decided for, but that means block-wise operation in an FSK case, since your system is not linear in modulation pulses, but has memory.


TL;DR: Don't. Your system design implies the channel is flat, and hence you don't need a channel estimate. Spend your time on something else!