Okay, let's take this idea seriously for a minute.
First off, if you want to play TMNT on Game Boy, play it on Game Boy. Nobody but you would really care if it's on Game Gear at this point. So if you really want to devote yourself to this project, be my guest.
Now, assuming you DO want to do it. Let's compare the two systems.
The Game Gear is almost identical to the Master System, aside from a lower resolution and substantially more colours. It uses a Z80 CPU and has 8KB RAM. The Game Boy has a custom CPU that is very similar to the Z80, and also has 8KB RAM.
Therefore, a lot of the main code can be ported over directly without much change, since the languages the CPUs use are almost the same. There will be no problem with RAM usage, and both systems have an identical resolution of 160x144, so the graphics will port over perfectly. Of course the sound hardware is rather different, but you could adapt the tunes somehow (it's the least of your worries).
So, is it impossible? Not at all: in fact, it's remarkable how similar the two systems actually are. But it would still take a lot of work - the underlying code may be copied over, but the code regarding graphics and sound would need extensive rewriting. For someone with experience in Z80 assembly, this would be a fun challenge, but if you're more interested in the result than the process, you'd be better off not bothering. In fact, that sentence could apply to all ROM hacking.
Now, the second idea.
they would have to be completely redrawn and colored in.
Coloured in, yes, redrawn, no. As I said, the resolution on both systems is the same, so unless the sprites are the wrong size, you won't need to redraw them. But yeah, importing the graphics would be orders of magnitude easier than porting the game.