The motor current changes significantly depending on the physical load as well as the supply voltage. 1A should be adequate to run both motors under the worst case conditions (12V, full power, physically stalled).
The Raspberry Pi current draw could be anywhere from maybe 200ma up to maybe 1.5A (depending on the RPi model, connected devices, etc.). Since the RPi is on the 5v rail being powered by a switching regulator (“buck regulator”), 1.5A at 5v would be 7.5W, which at 12v would only be 0.625A (about 0.7A factoring in the protection diode voltage loss and the efficiency of the regulator). 1.5A is higher than I have ever measured a RPi 3 to draw, even with WiFi and Ethernet connections, so it should be a very safe number.
Each sensor connected to the GPG3 draws probably 0-100ma (depending on the sensor) on the 5v rail, which is negligible. The GPG3’s onboard circuitry, LEDs, etc. are also minimal; maybe 150ma max on 5v.
Servo current draw under typical operating conditions should be no more than maybe 250ma each, but it depends significantly on the servo used. These also run on the 5v rail.
To add that all up, that’s almost exactly 2A on the 12v rail, and a lot of the numbers above are pretty much worst-case scenario.
The barrel jack on the GPG3 is indeed the standard 5.5mm X 2.1mm.