I tried to compile a 7.x Version of gdb without any luck. I codesigned the executable(http://sourceware.org/gdb/wiki/BuildingOnDarwin).
With following version there
i installed gdb 7.5 on mountain lion without problems...
those steps might help you: ./configure --prefix=/usr/local --enable-targets=x86_64-apple-darwin10 --enable-64-bit-bfd --disable-werror --build=x86_64-apple-darwin10 --host=x86_64-apple-darwin10 --target=x86_64-apple-darwin10
then make install gdb.
create your own certificate like discribed here and sign the gdb http://sourceware.org/gdb/wiki/BuildingOnDarwin
you have to explicitly state: codesign -s gdb-cert /usr/local/gdb before you sign the gdb make sure you already set your certificate to trusted. also close the keychain before signing your gdb.
hope this also works for your