Gulabo occupies a very lowly place in the Hindu caste system. A humble washerwoman, she's certainly no Brahmin, although one could be forgiven for thinking she was due to her imperious manner and snobbish sense of superiority.
Gulabo cordially detests Bakha and his sister Sohini, as they come from the lowest caste, the Untouchable caste, the caste that performs all the dirty jobs in society such as cleaning toilets. Gulabo is also fiercely jealous of Sohini's beauty. For someone as caste-conscious as Gulabo, it's simply outrageous that a woman of the lowest caste should be so beautiful.
So when she sees Sohini approach to fetch water, she lets rip with some really nasty verbal abuse. Gulabo is so full of hatred towards Sohini that she thinks of her as less than human. At one point, she even calls Sohini an “eater of dung and drinker of urine”.
As one can imagine, Gulabo doesn't have much time for Bakha. He, after all, is of the same caste as Sohini, and so equally contemptible in Gulabo's eyes. Gulabo deeply resents the fact that her son Ram Charan spends so much time hanging out with Bakha.
As well as hating Bakha for being of the lowest caste, Gulabo positively loathes him for being a bad influence on her son, for encouraging him to skip school. It's hardly surprising, then, that Gulabo doesn't invite Bakha to Ram Charan's forthcoming wedding.