RCS is partially open. The protocol is public, as are one or two authentication features. The biggest restriction for custom RCS implementations is that carriers often require access to SIM card functionality to authenticate a phone to their IMS. Only system software (your OS vendor, Google, maybe additional libraries like Facebook in some products) can access those. Unlike SMS, there is no simple "send this string to the modem and you've sent a message" communication method. The entire thing is SIP+RTP+a few other protocols, wrapped up in a brandable package. RCS is as open as SMS has been for a while, perhaps even more open as the SMS stack can only be reliably implemented in IP-only software stacks in LTE+ networks; 3G and below require integrations not even the Android system supports natively.

If you run a custom ROM, you can sign your own RCS app and have no such restrictions, of course. The same is true for devices with root access. There's nothing preventing anyone from writing a fully featured RCS client or library for custom ROMs, except maybe carriers filtering out unofficial ROMs, but those are a SIM card swap away. Nobody seems to have started working on an RCS app for those platforms yet. There are a few open source libraries out there, but they don't see much activity, and none of them implement the full RCS suite (which includes video calling and even exchanging money).

When people complain about RCS being closed off, most of them don't care about the RCS protocol. They want Google's libraries to handle all the hard work for them and provide an API to interact with without having to implement the carrier protocol side. RCS is already open, but they want Google Messages to be open, not RCS.

However, the EU's laws regarding gatekeepers require that a significant amount of people actually use the supposedly gatekept platform. Very few people within the EU use RCS. I don't think RCS is even close to being relevant for the EU's gatekeeper regulations. The only people talking about RCS on the European market are companies trying to peddle their RCS marketing spam delivery mechanism to other companies.

If the USA would adopt similar laws, the situation would probably be different. Don't expect the EU to care about gatekeepers in a market consumers aren't interested in.