To manually adjust the z-offset is to first auto-home your gMax. Then go to Menu>Prepare>Move Axis>Z-axis>0.1 mm and start to move your hotend down. Place a piece of paper between the hotend and the print bed. When the nozzle starts to get close to the print bed, start to move the piece of paper with your hand at the same time. Once you feel the piece of paper getting pinched between the hotend and the print bed, take a look at the LCD screen. You will want to write down what ever number is show.
Once you have that number, you will want to go up a couple menu's. Then you will want to navigate to Menu>Control>Motion>Z-offset. There will be number that represents your current z-offset. You will want to add that number to the number you wrote down after the auto-home. An example of this would be -1.9 + (-0.04) = -1.94
Once you have this number entered, you will need to go up 2 menus to the Control Menu and scroll down and click "Store Settings" to save the new offset.
If you are using our latest Marlin 1.1.8 firmware, you can adjust the z-offset with the babystep z function. Once you start a print and babystep z to the correct height, navigate to Menu>Control>Store Settings. This will store the babystep adjustment to use on future prints.
The way to physically adjust the BLTouch height is through the screws mounting it to the bracket attached to the extruder. Tightening the screws will raise the BLTouch and loosening them will lower it. One full turn with a screw driver will be about a 0.3 mm change in the z-offset. Make sure to adjust both screws equally.
With the exception of the Easy method, you should run the level bed operation inside of the gMax Tools menu after any adjustment.