I heard this phrase recently, and it took me back to when I was at University. Our college CU was run by a Calvinist Baptist, who was a lovely guy, but had a lot of concern as to who was 'sound' and who was not. Personally I have always been a bit more relaxed about that and more concerned as to whether people were helpful.
The phrase 'tell the gospel' implies one way traffic, and for many of my university CU brethren (the people who worried about this stuff were almost entirely male), that was what it was. Correct doctrine meant salvation; accuracy was everything. "Is he sound" was the key question re speakers. I always thought the more important question was "is he/she wrestling with what God wants to say?"
Maybe I'm becoming a liberal softie in my middle age; no I've always been like this. I prefer phrases like share the gospel, which implies the other person might have something to say. And it depends whether by gospel you mean Gospel.
Let me explain. Gospel means good news. Gospel, godspel, good spiel (remember that GCSE German?) The writers of the 4 gospels saw their entire narrative as the good news. If by telling the gospel, you mean tell the story of Jesus, I'm with you all the way. The Biblical authors saw the story as having power in its own right. Presenting Jesus to the world is a task I'm happy to sign up for. It's good news for the world.
However, if you mean gospel = a set of doctrines which come from a specific perspective (in the case of the speaker I refer to, probably a Reformed Protestant standpoint) then I have a problem.
We need some humility in advocating whatever we understand to be the truth; after all I have been known to be wrong.