Envelope budgeting with credit cards

[Mister Ribble]

I'm considering using gnucash for my budgeting. My budgeting is a little 
unique, but I imagine others have found a satisfactory solution - 
hopefully they'll chime in.

My finances revolve around a single checking account and a single credit 
card. Each month I allocate a number of income dollars to various 
budgets, say 100$ for groceries, 200$ for gas, etc. These allocations 
roll over each month, so if I spend less than the allocated amount, the 
next month I'll have 200$ in gas + whatever was left from last month. 
This way I can build up funds for things like car repairs that happen 
randomly.

All of my transactions go through my credit card, which I pay off at the 
end of the month.

I haven't been able to figure out how to move my money around in 
gnucash. If I move money out of my checking account and in to a 'cash' 
account for groceries or gas, then whenever I charge something to my 
credit card that should be groceries or gas, I end up with the allocated 
amount plus the spent amount, rather than allocated - spent. I've tried 
fiddling with different account types, but can't figure out how to get 
the accounts to add up. Any suggestions?

-Eli Ribble