There are really two things to consider: how large a gate the modules output and how large or small a gate they will accept as a gate input.
I’d say 5 V is a good gate level to generate. Most other synths will accept that as a gate and it won’t be too large for most synths. Maybe say output gates should be in the range 4.0–5.5 V?
As for gate inputs, there’s no reason not to accept gates down to a threshold of a volt or two.
Ideally gate inputs should allow and be protected against at least 15 V — I think Ken Stone’s designs, for instance, can put out gates close to that, see CGS gate sequencer - Synth DIY Wiki .