I2C: Electrical

I2C is very simple, you only need to master it

0V and 3.3 Volts. Or whatever
Just agree about the positive level with your peripherals.
No special impedance requirement (lines are short anyways)
The timing is driven by the master, no special requirement
Only, don't be too fast

You can drive I2C from GPIO
It works if your GPIO pins are open-drain.
But it also works if you can switch between input and output-low 

You usually can't be an I2C slave through GPIO
You need to obey master timing choices

If something goes wrong, the oscilloscope helps, as usual
Even a TDC (a logic analyzer) can help, but not for all bugs.