If we were to dismiss the thematic unlikeliness of any of these characters to start channeling and look at the math,

Gawyn and Mat would be the strongest cases to make out of the two since they both share a sibling who can channel. That said, knowing that the "channeling gene" is a recessive, even if you were to look at it as a simple pairing of a single gene inherited from the parents:

 

Channeling gene: "c": Dominant "C" inability to channel, recessive "c" ability -- ability to channel manifests with "cc" pairing

Two carrier parents: Cc -- Cc

Possible offspring gene combinations: CC, Cc, Cc, cc

 

there'd be a 25% chance of each child inheriting the (cc) twin recessive genes. It's a 1 in 16 chance in this scenario for two siblings to both inherit the ability to channel.

 

Now, obviously it isn't as simple as that since there's also differences between "sparkers" and "learners" and channeling is also tied to the soul. It simply hasn't been explained further than that; for instance, are there more than one gene that controls channeling ability? And how do you quantify the ties of channeling ability to the "soul?"

 

That said, the genetic possibility for Mat or Gawyn to have inherited the ability to channel is there and not out of the realm of possibility. I highly doubt we'll ever delve into it within the context of the series, however.

Actually that way of looking at genetic inheritance is not supported anywhere anymore. It's completely wrong aproach and was deviced before generic inheritance was understood. So you could actually dissregard it compleatly since it has no bearing at all on anything. =)

The way genetic inheritance happens is a much more complex procedure. Don't ask me about the chance of one of them being a chaneller though since I have no idea of how you actually should calculate it.