I've always found their dynamic more about a twisted form of respect than pure rivalry, honestly. The trust isn't given; it's extracted through conflict, like two predators circling each other who know a direct fight would be mutually destructive. Roberta's code and Garcia's inherited position force them into this weirdly formal dance.
There's a specific fanfic I read years ago, can't even remember the title now, that nailed it. It had Garcia deliberately leaving a strategic weakness exposed during a tense negotiation, not because he trusted Roberta's goodness, but because he trusted her intelligence to see the mutual benefit in not exploiting it right then. That's the core of it for me—their trust is built on cold, calculated reads of each other's self-interest, not warmth. It feels more real and dangerous that way.
The rivalry aspect gets inflated sometimes. They're rivals for control, sure, but not necessarily for the same throne. Garcia often seems to be fighting for a system's stability, even a flawed one, while Roberta fights for a principle that would burn that system down. Their clashes aren't personal animosity so much as the inevitable collision of two opposing life philosophies forced into proximity. It's less 'I hate you' and more 'your entire existence is a problem for my world,' which is a far more interesting tension to explore.