Unfortunalety you will not be able to access the D+E terminals with your United boarding pass, as they are not connected to the G, H or J terminals. You will get access to the South Terminal (from which United operates), but not to the North Terminal (from which American operates).

United operates from the H terminal (part of the South Terminal), which is connected to the J terminal. Both are renovated and nice looking. The best places to eat and kill some time (apart from the lounges) are in the corridor that connects H and J.

I have not been in the G terminal in eons and have no idea about its current state, but I believe only very small airlines operates from there.