Gtalk is an IM client. But it could easily become a so called "group messaging" client like BBM, iMessage, Beluga, etc. It needs group messaging for a start, saved histories (not just to your Gmail but ones that always appear in the client even if you reboot, etc), and it needs file sending (which I believe is present in the web/desktop clients already).
But yeah, they have the obvious advantage in this area of it already being tied to Gmail.... far more people have Gmail and always will compared to crappy platform restricted bullshit like BBM/iMessage.
It's about perception, Gtalk just needs a reinvention. People perceive these BBM/KIK/WhatsApp "group messaging apps" as being different to traditional Instant Messengers (like Gtalk) but really it's mostly common ground between them. Gtalk has video chat however, which traditionally isn't a feature of group messaging apps, but I don't see why it shouldn't be. Whatever is the easiest but most customizable application to suit your needs would be the best one.
Anyway, Google has been building a group messaing app for some time. I believe it's US-only right now, but it's called Disco -
www.disco.com and it's already available on both Android and iOS. But I think it's US-only right now. This product wasn't originally internally developed at Google I don't think, but Google acquired the company early on and has allowed it to continue autonomously up to now. I figure they just merge it with Gtalk and perhaps give the combined product a new name and it could dominate this market.