I have an acquaintance who is like Gemma in several ways, being a larger than life character, attention seeking, opinionated , overweight, meddling etc. She is very competitive, hyper-critical of everyone, especially other women, all other women in fact. Most people think she is common, a cow and a playground bully. But she is also great fun and oddly very kind and generous to lost souls and those struggling. She is definitely very entertaining if not always in a good way.
My analysis of her, and so also of Gemma, is that she actually is the opposite of what most people see. She suffers from low self-esteem and feelings of failure - she's on her own after several failed marriages - and hides it with inflated 'arrogance'. She imagines everyone is saying the worst about her so she says nasty things about them. To check what they feel about her she is confrontational almost as if to force others to say their worst. She encourages cliques, like a bully. She is very defensive and actually insecure but covers this by getting her attack in first. She cheats in games because she knows she is less well educated than most of her friends so she is desperate to win and come first. She is very conscious of her weight and jokes about it loudly to defer any jibes she imagines might be targeted at her, ie she's already said worse about herself. But I can see it depresses her.
I would say that that is mostly true of Gemma and that inside she is probably a scared little girl battling with the world. I'm afraid though, that does not make me like her.