You did not show it in your error message, but when gcc48 failed to install, it should have printed the location of gcc48's main.log file. (If it did not, you are using an outdated version of MacPorts and should upgrade to MacPorts 2.1.1 by using "sudo port selfupdate".) You should attach that log file to this ticket so that we can see why it failed. You can use the command "port logfile gcc48" to see where it is.
gcc48 depends on ppl, so if an update of ppl was available, MacPorts would have upgraded ppl first before trying to install gcc48. If that increased ppl's library version, you would now also need to rebuild any other ports that use ppl, such as gcc47. (On my system, the ppl port installed libppl.11.dylib, and no longer the older libppl.9.dylib that your gcc47 says it is expecting.) After running "sudo port selfupdate", use "port outdated" to see if gcc47 and any other ports have upgrades available; if so, upgrade them using "sudo port upgrade outdated".