You can power the kit using a variety of sources. The analogue circuit is powered from the 3.3V output of the voltage regulator on the ST Nucleo board, therefore the voltage on the GM tubes will not be affected by the input voltage level, provided you power the kit as described below.
The input voltage options for the ST Nucleo are described in the ST Nucleo datasheet page 17, section 6.4.2 for reference.
The power jumpers are JP1 and JP4 on the kit.
JP1 is located under the ST Nucleo board, so you will have to remove the micro controller to change the power options. This helps to maintain safe lab practices, too.