Simply said, Americans may be really familiar about PST and EST, all those. But the rest of the world finds it impossible not to first look up the definition before doing maths. Compare:

1. Wild PST / EST appears
2. Check the Internet to see which time-zone corresponds to which time-zone we have, etc.
3. Do maths.

And

1. Wild GMT±n appears
2. Do maths.

Of course, we shouldn't enforce anything on chats but we should simply use more GMT±n as it is just simple maths for the whole world to understand. I suppose we should probably use GMT for all the event organisers so participants are easier to know the deadline or something.