I have tested the current consumption at 5W Tx and its 1.3A. This is much less than the MD-UV380 at 5W, which consumes 1.7A
Please reload the official firmware, and check the power output on the High power setting, and measure the current consumption of your radio e.g. using a PSU set to 8V.
Note. The output power on the High setting is not likely be 5W becuase the official calibration data in all supported radios is normally very inaccurate.
Then reload the OpenGD77 and use the calibration menu to transmit the same power, and compare the current consumption.
Battery consumption on Rx is dependant on the Eco setting. Default setting is Eco = 1, which an average level of economy, but may take more current that the official firmware, but the response time of the firmware is 100% better on Rx than using the official firmware
If you want slower response times and bettery battery life, then increase the Eco setting number.
Please read the User Guide for full details