Using digital input pin to read variable resistance from sensor requiring supply voltage
First, be aware that your sensor does require a 5v +-0.2 supply, both for the heater and sensor. The RPI should have +5v available.
The capacitor discharge method that you propose is one of very few available to RPI to convert an analog signal to a digital representation. RPI GPIO pins require a voltage range between 0 to +3.3v.
Data sheet says load resistor minimum is 450 ohms. No maximum, so a capacitor load might be acceptable:







simulate this circuit &ndash; Schematic created using CircuitLab

This circuit at left might work, if you let the circuit idle with GPIO as output, programming a logic zero forcing R1 to pull down the cell resistor R2 toward ground, until you wish to input a reading. Reading may go quickly enough not to disturb R2 cell resistance too much.
If you are reluctant to disturb cell operation from the prescribed load circuit, the amplifier at right might allow a similar capacitor charging/timing method. A quick run with LTSPICE suggests a timing range between 800 uS for a sensor resistance of 100k ohm, and about one second for sensor resistance of 10k ohm. Cell resistor R4 remains always near 5v.
However, be aware that the GPIO measurement sequence requires a polarity switch: C2 must be discharged by setting GPIO to logic high, then transistor Q1 pulls it down slowly towards logic 0