Heat? You do not want to place your speakers on top of a heat source. It could ruin the finish, start the grill on fire, burn the cones...
That being said, I have a speaker of mine "near" a natural fireplace (which gets far hotter than most gas units) and there is absolutely no problem. It is however, a deep fireplace, I keep the fires "small", and the closest the speaker gets to the surround is about four feet (I have no idea of your definition of "near") .
Bottom line, I would not challenge the proximities of speakers to heat sources...if you value your speakers.