Board Thread:What If?/@comment-5674726-20140809221200/@comment-24762954-20140810181137

ReganX wrote: Arthur wouldn't have known of Gaius' involvement and he'd expect that Gaius wouldn't know of any plan to rescue Mordred. Merlin would be the person he'd expect to help him with the escape plan, as arranged, and who he would also expect to keep the plan to himself.

And still Gaius was involved which he could tell Arthur. Since Merlin lives with him as his ward Gaius would notice that something is off and act accordingly. He didn't need to know what exactly was going on. If Gaius would speak with Arthur, not Merlin.