Having this as a member var won't work in a multi user environment.
If two admins are on the management page at the same time this value would alternate and be overwritten between each individual http request.

Either get the value from the http request or the session.

But If I was doing it I would restructure the management page instead. In a similar manner to how it is done in other parts of Jenkins.
By first sowing a "common" management page with a list of some kind of the different configured servers, on this page you also select to create a new server.
By clicking on a server the browser would navigate to for example jenkins/management/gerrit-trigger/server/serverName/
in effect that would then be a call to "public GerritServer GerritManagement.getServer(String serverName)" and GerritServer has a index.jelly view with the configuration form.

That way you always know what server the current user is configuring because you are actually running code in the correct object instance.

I have done it in similar ways in both the Build Failure Analyzer and Team Views plugin for reference.

WDYT?