I would like to have the GUI and the functions which are within it which the reference manual points to them,
such as ad9361_tx_quad_calib, ad9361_init, etc...
Go to this link and select the evaluation platform you want to use. Click on a corresponding link and you will be taken to a resource download page.
Introduction to boards based on the AD9361 [Analog Devices Wiki]
In case you have driver or GUI related questions we have specific forums that deal with those.
Can you point me to GUI related forum?
Yes, see link below:
Linux Software Drivers
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