You might think about using high speed digital logic, e.g. 10 GHz ECL drivers, programmable attenuators, a DC offset source and a resistive combiner to generate the intended waveforms.
A high speed DAC or dedicated analog circuits are the other option. Besides rise and fall time (usually defined for step 10/90% level) more signal quality criteria (overshoot, magnitude accuracy) might be necessary. "Appear square" sounds reasonable at first sight but is actually a rather vague specification.
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In the usual second order system metric, 0.2 ns rise time relates to 1.75 GHz bandwidth.