If you go to the Audio settings in-game and you see an ! sign which states your microphone permission has been denied, you will need to enable it to use your mic again.
To fix this please hover over the Gun Raiders app icon in the home screen and look for the 3 dots and click on the 'Permissions' tab. Once there, make sure the permission button is enabled and blue to "record audio" for the microphone. You may need to toggle the button off and back on to reset the permission fully.
You will then need to make sure your mic is enabled on your headset settings and in the in-game settings in the pause menu.
If you still have issues you may uninstall and reinstall the game and it will prompt you to allow the mic to record audio after you leave the tutorial for the first time.