Yes, you need to buy the gas hob before installation of the modular kitchen as the person or company whoever is installing or making the modular kitchen will cut the top part of the carcass along with countertop of that particular area as per the exact size of the hob so that it can be fixed there perfectly. If you won’t buy the hob before the installation of the modular kitchen then certainly there will be adjustment problem of your hob in the hob unit carcass.