Can I use gpio pins to transmit analog signals? And what is difference between this io_in and gpio_analog?

I have the same question

io_in

is the 1.8V digital input from the pad. And the answer is yes, you can use the GPIO pins to transmit analog signals by (1) connecting the signal to

gpio_analog

and (2) configuring the GPIO in software to turn off both the digital input and output buffers. If you cannot tolerate the 150 ohm resistor between the pad and the signal, you can use the

gpio_noesd

signal instead, but be aware that the connection is very ESD-sensitive.

is there any way to hard wire those output buffers off without needing to use the management CPU? eg hardwiring oe to ground

Well, all GPIO pins will come up in a state of being a digital input, so it would not interfere with an analog signal other than drawing some current if the signal is mid-range between power and ground. In other words, by default the output buffers are off, and only the input buffers are on. Possibly it would make more sense for most of the GPIO pins to default to a completely disabled state (both input and output buffers off) on power-up.