GDB couldn't find real strings most of the time. But, you can find the offset of the string from the local object file and adjust it with dynamic loading information.
From info proc map:
Mapped address spaces:
Start Addr End Addr Size Offset objfile
0x8048000 0x8049000 0x1000 0 /opt/protostar/bin/stack6
To write the string "foo" into the memory address 0xdeadbeef:
w foo @ 0xdeadbeef
To write the hex 0x41414141 to the memory address 0xdeadbeef:
w \x41\x41\x41\x41 @ 0xdeadbeef
I recommend also taking a look at the various options for writing using the command w?.