I have just bought a cheap cycle for regular use (Kross Bolt 26"), but it comes in single speed. Can I put a 1×6/7 gear set in it?....the same model (28") comes with 21 speed gear set-up. How much will a cheap gear set cost?
Technically yes, economically no.
You obviously need to obtain a multi sprocket cassette, rear derailleur, shifter, and cable and housing. Additionally you need:
Singlespeed frames have dropouts designed to provide a mechanism for adjusting chain tension by moving the rear hub, so you would need dropout adapters to fit a rear derailleur.
Single speed frames lack shifter cable bosses, so you have to get bolt on ones.
Those might be difficult to obtain. The real problem is that many other major components would need to be replaced:
You would need a new rear wheel with a hub designed for a multi sprocket cassette.
Derailleur chains are narrower than single speed chains, so you would also need a new chain
Because you have a narrower chain you need to replace the chainrings, which as this price point are integral with the cranks and cannot be swapped out, so you need a new crank.
Basically, you have to replace too many of the bike's original major components to make it worthwhile.
You could install a hub with internal gears. You would avoid the derailleur/chain tension issues this way. Your bike would still lack bosses for the shifter cable, but you could use zip-ties or cables clamps here for support instead.
I can think of three issues you might have however. First is price; internal hubs are not cheap. Is your current bike worth throwing money at? Second is dropout spacing. You would need to measure the rear dropouts and find a hub that fits. Third is chain line. With your existing cranks and a geared hub, would the chain line be acceptable, or would the bottom bracket need to be changed up front?