Step 1: document the situation. Put up a notice that explains the problem; but be clear, not alarmist. As discussed on other questions, there's no problem for users of your plugin, only for people who would want to distribute a combined work of your plugin and the host.
Step 2: Survey the contributions of the unreachable individuals. Perhaps they are very small? This might either make them easily replaceable, or not big enough to copyright.
Step 3: Survey the mailing list archives. If there's a clear intention to allow this usage, then you might conclude that it's safe to change the license (by adding a permissive term or by switching to LGPL) on the grounds that you have evidence that all the copyright holders intended this in the first place.
HOWEVER: You've noted in other questions that you incorporated code from other GPL sources. You can't relicense that without permission from its copyright holders, so there's really no point to changing your license unless you can replace that code.
To repeat what I have written elsewhere: You and your fellow contributors have not violated anything. You have, however, created an opportunity for a very small group of people to violate the license of the GPL code you incorporated. If there is no realistic situation in which anyone would want to distribute a combined work, then this is more humorous than disastrous. Your notice would consist, more or less, of:
Hey, we used the GPL and we forgot to add a dynamic linking exemption, and we
incorporated some other GPL code that has no dynamic linking exception.
If you are an end user, this makes no difference to you. The GPL
gives you all the rights you need. Our mistake only matters if you
want to create a derived work by combining our plugin with <INSERT
APPLICATION NAME HERE> and distribute that combined work. In that case, we're really
sorry, but we can't guarantee that some copyright holder won't call
you out for a violation, so we have to tell you that this would be a
very bad idea.