I'm a big supporter of the GCP plan, but recognise that it has some serious problems.
Primarily it's an attempt to solve one problem (congestion, car dominance) with another solution: new funding.
The trouble here begins (as it does with so many things) with Tories starving the cities financially. We need better transit infrastructure to support people getting out of their cars, but there's no cash to do it, so we create a congestion charge and use that money. That's why it feels like a cash grab: because it is.
However: what most people don't know is that this was coming regardless. The petrol tax revenue is drying up and nationally, governments are looking at doing this everywhere to compensate. On top of that, cars are wildly subsidized by all levels of government in the form of road maintenance, parking, air quality, noise pollution, and straight-up deaths, so a £5 charge is actually a correction rather than the abuse some have tried to suggest.
The GCP plan isn't perfect, but it's a step in the right direction towards better transit and fewer cars. I'd prefer them to be more aggressive though: nationalise and integrate the buses and close more roads to cars thought the city -- but if they haven't found the will for that yet, this will do.