Can an increased current rating of a Ground Power Unit damage an aircraft?
I find the easiest way to think about Volts and Amps is that the voltage is "pushed" and the amps are "pulled".

This is the same with pretty much any kind of power supply, whether we're talking mobile phones or planes.

To that end, the voltage requirement always needs to match. If your device is expecting 28Volts in DC then your power supply MUST provide 28Volts in DC - anything else is likely to damage it, could cause a danger and almost certainly won't work.

But current (Amps) is different - your power supply needs to meet or exceed the current demands. So if your device can pull up to 1000 Amps, the power supply MUST be able to provide 1000 Amps or more.

So, to answer your two possible questions. You can increase the Power Supply (GPU) to a 1200 Amp model (or beyond) with no ill effects, but your device (The aircraft) must have a current demand that meets or is less than that.