The majority of the UK is supplied by the national grid for their gas supply. An IGT means an independent company manages the pipes that supply your home. This is often the case in rural areas and newly built properties.
You can find out if your home is supplied by an IGT by checking your meter point reference number (found on any old bill). If it starts with a 74, 75, 76 or 77 you are one of the special ones!
In the olden days, you may have had to pay more or it might take longer to switch, but thanks to a new industry-wide project called Nexus you shouldn’t experience any difficulties.