Q1: Can I do that in LimeSuiteGUI?
A1: Yes. It would be best if your would do it via LimeSuiteGUI.
Q2: Should I do that while the enb is running or once the enb program is closed?
A2: Depends on your enb program. If it controls LimeSDR (which it should), the communication channel will be occupied by it and LimeSuiteGUI might not connect to board (USB line will be busy). It would be best to disable the enb program, but make sure that it does not alter the last state (reset at disable);
Q3: Once I reset my computer and decrease tx_gain to 30, then it works again. What problem could that be?
A3: Please check you power supply capabilities. This is most likely related to either power limitation or faulty USB cable.