To determine which lamp gives nearly monochromatic light, we need to understand the characteristics of the light produced by each type of lamp mentioned in the options.

1. Sodium Vapour Lamp: Sodium vapour lamps emit light primarily at a wavelength of about 589 nm, which is in the yellow region of the visible spectrum. The light appears almost monochromatic because it consists mostly of two closely spaced wavelengths (the D-lines of sodium: D1 at 589.6 nm and D2 at 589.0 nm). Thus, sodium vapour lamps are known for producing nearly monochromatic light.
2. GLS Lamp (General Lighting Service Lamp): This is a typical incandescent lamp that emits a broad spectrum of light, covering a wide range of wavelengths, producing white light. Hence, it is not monochromatic.
3. Tube Light: A tube light, commonly known as a fluorescent lamp, also emits a broad spectrum of light due to the phosphor coating inside the tube, which converts ultraviolet light into visible light of various wavelengths. Thus, it is not monochromatic.
4. Mercury Vapour Lamp: Mercury vapour lamps emit light across a range of wavelengths, including some ultraviolet and visible light, resulting in a broad spectrum output. This makes them unsuitable for monochromatic applications.

Based on the characteristics above, the sodium vapour lamp is the correct choice for providing nearly monochromatic light. This is because it emits light predominantly at two very close wavelengths, making its light almost monochromatic.