a free standing cooker must be stabilised against tipping over by either a safety chain, which is rawlplugged at one side into the wall, available from a plumbers merchant.
the preferred method is a stability bracket again screwed into the wall or the floor.it is u shaped and must protrude into the cooker engagement slot by 90mm.all cookers produced in the uk have a stability bracket engagement slot which is the easier method and safer because you can adjust the gap between the underside of the bracket and the bottom of the slot to have the minimum amount of movement whereas the chain needs slack to pull it out a bit to enable you to reach along the greasy side and unhook it. the chain is short you see .
is the cooker new or second hand? there are a number of safety checks to carry out, gas leak testing ventilation in the room, combustion air requirements,,flame safety devices, shut off valves, proximity to combustible surfaces (never enough allowed by kitchen installers) etc. these need to be carried out by a corgi registered fitter and i presume that you do not have this knowledge due to the basic level of your question
please go down the safe route and do it right
good luck