Question: I am looking at establish a circuit allowing to detect when one or two wire are cut such as a tamper system. When the wire are cut it should signal this a GPIO.
I suppose I need to use a transistor connected to GPIO but I have a bit some difficulty to see how.

Answer: 
An alternative to detecting change in voltage on the GPIO pin would be to use a dedicated [current loop](https://en.wikipedia.org/wiki/Current_loop) controller. There exists many [4–20mA](https://electronics.stackexchange.com/tags/4-20ma/info) control circuits that will have a indicator pin for when there is no current flowing which you can monitor.
It depends on what level of tampering you want to be able to detect. The minimum approach is described by transistor.

Note that a clever tamperer can simply ground the top line, then cut the loop and perform mischief. A more robust system uses two pins. The processor drives one with different values, then checks the other to verify the new output value is detected. For extra credit, you make the values pseudo-random, like what comes from something like a CRC calculation.

However, all this only describes the logic, not the real implementation. I would not just want to connect processor pins directly to external wires, especially when tampering is a possibility. At least any processor input should be protected. CMOS inputs are high impedance, and there is no reason to require high speed, so 100 kΩ or so between any external connection and a processor input is a good idea.

Depending on the level of abuse you want to be able to live thru, you may want your own clamping or rely on the built-in clamping circuitry of the processor. The processor may not be specified to operate correctly with current thru its clamping circuitry, but it should survive.

For good tamper resistance, I'd say that external lines should be able to tolerate being connected to whatever line power is where the device will be used. That means 120 VAC here in North America, for example.