GNU Radio GMSK modulator
Let's say you want to filter a signal $x[n]$ through a Gaussian filter with impulse response $h_g[n]$ and a Moving Average (the "sqwave") filter with impulse response $h_s[n]$.  Then the resulting operation is

$$y[n] = h_s[n] * (h_g[n] * x[n]) = (h_s[n] * h_g[n]) * x[n]$$

So the a priori convolution of the two filter tap sets into one filter taps set, just reduces the two filtering operations into one filtering operation.

The Moving Average filter is just a low pass filter with a poor transition band and poor sidelobe supression.  (The one used here isn't even normalized to unity gain.)

I can't definitely answer "why?", except to say that it looks like someone wanted to add a cheap low pass filter as a sort of channel filter.  That moving average filter has been there since 2007, when GNURadio's git history starts.  The old CVS repositories aren't on-line anymore, so the original rationale is probably lost to history.