Both Talking Beast and Leaf with Small Beast and Leaf allow you to communicate in a limited way with creatures that normally would not understand you, but they work differently and do not imply that communication is secret.
Even if you ignored the fact that firbolgs and gnomes are neither animals nor plants, these traits would basically do nothing to improve communication between them.
Consider talking to little animals:
With the help of sounds and gestures, you can communicate simple ideas to small or small animals.
If someone does frantic and repeated gestures in a certain direction, I will finally understand that, for compelling reasons, I have to go in that direction, but a rat will never do it (unless it's over). Act of a trained rat). Whereas if a forest dwarf does the same, even a rat can understand.
Talking to Small Beast does not do anything for the forest, because people are already able to communicate simple ideas to each other through sounds and gestures. None of this is particularly subtle or secretive, and anyone in search would understand the essence.
Now consider the speech of the beast and the leaf:
You have the ability to communicate in a limited way with animals and plants. They can understand the meaning of your words, although you have no particular ability to understand them in return.
If someone yells "You! Go!", I can understand the basics by understanding English well, but a dog or a cactus will never understand (unless it's a dog trained). While if a firbolg does the same thing, even a cactus can somehow understand.
Speech Beast and Leaf does not do much for the communication firbolg to gnome forest. If the firbolg uses a language that the gnome understands, he can already communicate. If the firbolg uses a language that the gnome does not understand, then Speech of Beast and Leaf would still allow limited communication. In any case, it would not be particularly subtle or secretive, and anyone listening would understand if they knew the language used by the firbolg.
But you know what else allows limited communication? A few days of preliminary work to find code words and unique gestures. It would not necessarily be subtle, but at least it would be obscure for others.
Passing this preliminary work by hand is entirely the responsibility of the DM.