I dont know everything there is to know about the code stuff with deltarune/undertale, so forgive me if this turns out majorly incorrect due to something I forgot, this theory is just for funsies.
So, hot take, i don't think Gaster is the man who speaks in hands. I know, scandalous. I don't think the font/cipher he speaks in is solid connection(yet). My theory isn't solid at all either sooo. I think the man who speaks in hands is the man behind the tree, and I do not think the man behind the tree is gaster, they speak differently. I was thinking about the use of the word man, it's not a word I recall showing up in either game a whole lot. I am curious to see how many times the word "man" is used, but college isn't even giving me time to eat or sleep, so I won't be able to get to that soon.
My point is, it stands out to me, as opposed to any other phrases to narrate a being we cannot see at all like "someone", "somebody", something, "person". This goes for the well known phrase "beware the man who speaks in hands" there's that word again man. Could this "man" perhaps be a human? It would explain the narrative choice for the man who gives us the egg to be behind the tree out of sight.
But what about the other part of that phrase you ask, we don't get anything about the man behind the tree's hands, but my theory continues. For the man behind the tree to be behind the tree in that position, could imply some sort of awareness to us the player, and our point of view. As for the hands, I think the man could be signing to Kris as he speaks, to not make us, the player suspicious. "Beware the man who speaks in hands" could be warning the player, not Frisk as they do not encounter anyone who speaks in hands on any of their journeys. The egg may be an incentive for the curious player to seek the man out. Being behind the tree would make sense if the man is signing to Kris. They could be conspiring against the player, or just distracting us from progressing, as finding the man always takes up a notable amount of time.
That's it for now, I'll update if I have any more thoughts, thank you for reading :D