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: 
![schematic](https://i.stack.imgur.com/A8UqT.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fA8UqT.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

*Figure 1. GPIO normally pulled low by tamper loop. Cutting loop causes GPIO to be pulled high by R1.*

C1 will help eliminate any noise on a long tamper loop.
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.