What is the power generating capacity for an airport GPU? What is the normal current for GPU power supply for commercial aircraft and how much power do they need while on ground (in kWh)?

I have read that aircraft need need 28V DC and 115V AC - what would be the percentage between both modes of supply?

Would it by possible to power GPUs entirely by solar power?
Power requirements are dependent on airframe and configuration (see this question for a general idea). GPU's come in varying sizes for varying aircraft types. For example this GPU (arbitrarily chosen) can deliver: 800 amps continuous, 2500 amps peak. While this GPU can only drive 70Amps continuous. Others may be able to drive more and some may drive less. Are you asking about a particular application? The GPU's at the tiny airport I fly out of that we use to bump start the occasional Piper are no where near the capacity of what you may find at JFK for starting up a 747.  


  Would it by possible to power GPUs entirely by solar power?


Depends on the application but it would be unlikely to chose such a medium for powering a GPU the way things stand now (in terms of power requirements). You may be able to have a battery powered GPU that is charged by solar cells but that would only work well in the smaller scenario. Having a solar powered airport that can provide ground power is a whole different question.