It won't be with RCS. The "gatekeeper" criteria applies to interoperation between dominant technologies. RCS has very small adoption in Europe. If iMessage will make the cut it will have to integrate with Whatsapp, Facebook Messenger and Signal.

Edit: I should clarify, there's RCS the standard and there's Google's implementation. Google's RCS is too small to be considered for itntegration. The standard on the other hand would be nice, in an ideal world; however, merging proprietary networks into an open standard is a very high goal and goes beyond what the EU wants to attempt at this time. Instead it will let the tech owners achieve interoperability in any way they want and can.