why does homolytic cleavage takes place only in gas medium?

This is not true. Many compounds homolytically cleave bonds in condensed phases. However, this type of reaction proceeds better in the gas phase, because the reverse reaction is much less likely to take place. For example, in the case of halides (X2):

X2(g) ↔ 2X· (g)

The product, halide free radical, can diffuse away and react before recombining to reform the starting material. A chemical I use quite often in my job is benzoyl peroxide, which is used to make certain plastics. It cleaves the O-O peroxide bond in a homolytic manner as well, but in solution:

C6H5CO2-O2CC6H5 ↔ C6H5CO2·

The C6H5CO2· free radical can go on to react to make a plastic, but since it is in a condensed phase, the vast majority of the free radical recombines to benzoyl peroxide. However, just enough of the free radical reacts to make the plastic polymerization reaction go.