I was able to step through a simple Swift project and set and hit breakpoints with GDB, those thing worked pretty well. It could evaluate simple (integer) local variables. Globals and function names were pretty badly mangled, it looks like GDB has no clue about Swift name mangling at this early stage, maybe a future release of GDB will. Swift string variables are opaque, so you couldn't really see their contents, though you could drill down far enough to see their length.
LLDB integration will be a huge improvement for MacOS, but it is no trivial undertaking so for the time being, it still falls under the "future, hopefully next, release" status.