Example of R and G when $R \subseteq L(G)$ is undecidable
Given an $R$ and a $G$, one can easily construct a TM which outputs whether $R \subseteq L(G)$.

(Consider two TMs, one that just rejects and one that just accepts. If not the first, then the second TM would definitely be correct.)

What is not possible is this: Give a TM that takes as input any $R$ and any $G$ and accepts iff $R \subseteq L(G)$ and rejects otherwise.

Note that the same is true for the halting problem. Given a TM $M$, it is decidable whether $M$ halts or not. (The same two TMs mentioned above would do.) What's undecidable is the general problem of being able to decide whether a TM given as input halts or not. (A single machine that can take any TM as input and be correct on all of them.)