At least for the first stage you don't actually need to run the executable (under gdb or otherwise). You can just get the string with objdump:

    $ objdump -s --start-address 0x402400 bomb|head

    bomb:     file format elf64-x86-64

    Contents of section .rodata:
     402400 426f7264 65722072 656c6174 696f6e73  Border relations
     402410 20776974 68204361 6e616461 20686176   with Canada hav
     402420 65206e65 76657220 6265656e 20626574  e never been bet
     402430 7465722e 00000000 576f7721 20596f75  ter.....Wow! You
     402440 27766520 64656675 73656420 74686520  've defused the
     402450 73656372 65742073 74616765 2100666c  secret stage!.fl