Have full-sized AWACs been replaced by making Gulfstreams into military planes with electronic monitoring equipment?
A number of different aircraft have supported the military airborne warning and control (or AWACS) mission over the years–notably the Boeing E-3 Sentry, based on the versatile 707 airframe and used by several nations, and the Grumman E-2 Hawkeye, which is used for tactical airborne early warning for US naval fleet operations. A review of Gulfstream Aerospace Corporation’s website confirms that some of its aircraft have been used in military applications in the past and are currently being used in various roles, but the AWACS mission is not listed among them. That doesn’t mean it won’t happen; it just does not appear to be happening at this time.