Why a glowing gas gives only certain wavelengths of light?

Answer 1

The atoms of glowing gas emit the wavelength of radiations which lie in its spectrum. The same wavelength it can absorb because these are suitable for excitation pass through th gas

Answer 2

Fluorescent lights work by ionizing atoms of Mercury in the mercury vapor in the tube, these ions then emit UV light. The inner surface of the tube is coated with a mixture of chemicals called phosphors, which emit visible light when excited by this UV light. Each phosphor chemical only emits a very limited number of wavelengths of light, so to get what looks to us like white light at least three phosphor chemicals are used: one that emits mostly red wavelengths, one that emits mostly green wavelengths, and one that emits mostly blue wavelengths to match the responses of the cone cells in the human eye.

Incandescent bulbs instead produce a broad "black body" spectrum because they generate their light entirely via heating. This spectrum goes deep into the IR, spans the entire visible spectrum, then ends abruptly in the lower end of the UVA.