That's exactly what I was arguing, yes. That $y is important and $x is irrelevant.
> They can't opt out of the road part of the service charge and build their own roads.
That has no real bearing on the economics of the situation. Paying $x to the government for roads and paying $x to a private contractor for roads work the same way. That's why the net subsidy is $y.
If there is a tax that's applied specifically to gas companies that isn't directly paying for infrastructure they use, then that tax can be subtracted from the subsidies. But the gas tax doesn't fit that bill; if anything it undercharges.
The gas tax is only one of many taxes that these companies pay - you have to sum all their subsidies and subtract all their taxes. That's the point. If you're not doing that, comparisons with subsidies to other industries are going to meaningless.
How can it be a fair representation of their situation to only say they receive $1bn/yr in subsidy?
You're right to say that it's not fair to only list the $1bn. It should be put in context of the $100bn of normal taxes. But reducing it to "$99bn" is not a fair representation either. It doesn't tell you if they otherwise would have paid 100 or 200.
But $x is not a service charge. It bares no relation to the amount of anything used. They can't opt out of the road part of the service charge and build their own roads. The tax is not hypothecated. Whatever the rhetoric of politicians, it doesn't bare any resemblance to a service charge.