When your retired you can go on holiday at anytime
But even FT working people get annual leave, so it's not like Christmas is the only time in the year when family could get together. If the idea is that "But it's Christmas, which is a special time" - it's just as special for the GPs as for the younger generations, surely? Maybe they want to spend that special time of the year together at the seaside - not on a random week at another time in the year.
It's sad if they don't want to spend much time with their GC, and their attitude to the GM (his or her DM) does sound mean; but we don't know what she's like and what history they have between them. Some people have the idea that very old people are all sweet and innocent. A lot of them are lovely, but equally, there are plenty of really selfish, nasty and manipulative people around - and they don't magically become lovely through living for several more decades. OTOH, she may have been a lovely kind person her whole life and they're just very selfish or unpleasant themselves - we have no way of knowing.
Whilst most GPs are at least fond of their GC, and many want to be hands-on, a lot of people seem to forget the fact that they're their own people too, as well as GPs, and they don't always want the end of their lives to be solely dedicated to other people.
They put their own lives on hold when they had their own children (which they chose), but if they're now expected to do the same again, now they have grandchildren (which they didn't actively choose), given their age, there's no longer any option to 'put on hold' until the future. This is the latter part of their lives and they may or may not have a number more years left, but how many of them in good health and able to travel independently?