If you have the proper training to do this type of work you would not be asking.

To properly charge this system you need to recover refrigerant, braise on a schrader valve, evacuate with a vaccum pump, and recharge. This is because it is a hermetically sealed system that does not come with a service port schrader valve, unlike cars. So you need a couple grand in specialized equipment, unlike cars where you can buy the 20 dollar can of R134a and put it in.

Also, that machine is basically worthless, so it's not worth fixing.

Sorry.

You can put a line tap on the low side line from the compressor and then read it with gauges rated for R134.  The low side should be properly charged at 18-20 lbs in an air-over condenser machine with an evaporator fan.  This is foolish to do yourself as you can easily overcharge a compressor and if any gas is to removed you have to have EPA certification to legally remove it.  Have a certified vending refrigeration tech recharge it for you.

I have the same problem with my G- 380 smh. After purchasing it I heard that they are not good machines.