I understand what you're saying, but I disagree on almost every level of your argument. First of all, in your list of fire-related gradual growth cards you actually have more than one concept there.
First, there's rage potion and fire nymph. Since all nymph abilities are those of their element's alchemy card, I don't really think this even counts as two cards, but that's beside the point. This is not gradual, and it's not continual. It's one time, as well as being a bigger boost that any other card mentioned, including overdrive.
There's also the two ablaze creatures, fire spirit and fire eater. Sure, every creature with this ability is going to be linked to fire(the name is called ablaze, after all), but of only two creatures in the game only one of them is fire. The other is gravity, showing that even the "purer" ability is not solely based in it's own element.
Also on the list is steam machine(actually, you didn't name it this time, but I'm pretty sure you did earlier). Well, if you think about the concept, you'll realize fire isn't even an aspect of the idea. You don't usually use fire directly to create steam. You could, of course, but all you really need is heat. Heat is energy, and fire is only one of multiple elements that are energy.
Last, but not least come the growth creatures, lava golem and forest spirit. Of the four elements used here, only one of them is fire, and again, if you want to go into the concept it's slightly inaccurate. Lava is superheated melted rock. You might find fire near lava, but the fire is usually a by product of the lava, not vice versa.
All in all? I think gravity needs this, gravity can use it best, and gravity has as much claim to it as any other.