It's possible to get the information with any debugger, you just need to know the ABI. On amd64 with the SystemV ABI, its %rax for integer return values and %xmm0 for floating-point return values.
If you don't want to remember, you can write a macro for it.
GDB probably has something builtin for this, but I gave up trying to find features in GDB a long time ago.
[edit]
Stackoverflow seems to be of the opinion that GDB doesn't have a tool for automatically showing the return value of a just-called function. However if you step into a function and execute the command "finish" you will return from the function and print the value. Properly showing the return value with source-level (rather than instruction-level) debugging is a non-trivial feature to get right.
