The two PWM outputs on T100MD888+ are N-channel MOSFET type and they sink current when turned ON, but each is internally pulled up to power supply voltage (either 12 or 24VDC) via a red LED and a 5.6K pull up resistor. To control power supplied to a load, you need to connect the "+" terminal of the load to the positive of the DC power supply (e.g. 24V or 12VDC) and the "-" terminal of the load to the PLC's output and the load will be turned ON and off rapidly within the PWM period but the average power applied to the load will be proportional to the duty cycle. Whether this will work for your application will depend on the what's needed to drive your gauge.
The two PWM outputs on T100MD2424+ are P-channel MOSFET and at 0% duty cycle they are turned OFF but at 100% duty cycle they are connected to the +positive power supply voltage (24V or 12V DC). So at 50% the output is about 50% of the power supply voltage (in earlier post I made a mistake to say that it is 22V at 100%, which I have amended that post subsequently). This is more likely to work with a gauge that will display a reading proportional to an average input voltage between 0V and 24V (or 12V) DC.
To increase the current drive capability of the AO, you can use an operational amplifier wired as a voltage follower which is probably easier than using a MOSFET in linear mode. There are plenty of text book example of voltage follower OpAmp circuit . E.g:
http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm