Is this a remake of "Revelations: the Demon Slayer" for the GBC? Or different SMT game?
It's a bit complicated. The original version of Last Bible came out for the Game Boy in 1992; that version was never released outside Japan. In 1999, with Pokemon flying off the shelves and portable games involving recruitable monsters suddenly a very attractive sell, the Game Boy version was ported, with some changes, to the Game Boy Color. That version is the one that was released internationally as "Revelations".
This Game Gear version dates from 1994, long before the GBC port, so it's based on the original Game Boy version and doesn't have the changes from the later version. It also adds several enhancements and twists of its own.
The main changes that were made in Revelations compared to the Game Boy/Game Gear versions are:
* Obviously, it's in color, albeit very blocky and splashed-on compared to this version.
* The GBC version is much easier. All enemies give out 1.5x more EXP and money, and a few pieces of equipment have better stats.
* Most overworld sprites were redrawn in a more cartoonish style, I guess to try to draw in the Pokemon crowd.
* The Game Boy version had a "pathfinding" feature similar to the Lunar games that would push you along walls if you walked into them. The implementation was really bad and it frequently pushed you in the wrong direction, so it was removed entirely in the GBC version.
The major changes in the Game Gear version compared to the original are:
* There's a dash button.
* The pathfinding was fixed to actually work as intended.
* Various enhancements to the script, including two new endings very different from the original.
* Lots of art improvements; aside from being in proper color, all the monster graphics were totally redrawn with different, more detailed designs.
* As a consequence of the larger monster graphics, the Game Gear version can only fit two enemy types on screen at once, so the three-enemy formations that occasionally showed up in the Game Boy version were dropped.
* Various music changes. Some tracks were rewritten, while others are the same but used in different places from the original.
* Also, the GB/GBC versions will shut off random encounters after you beat a dungeon, but this one won't. Protip: Always carry a Sylph Wing unless you want to hike back out through all the enemies you fought coming in.
Basically, if you've played Revelations, this version is going to be harder and better-looking. And have an intelligible script, unlike whatever the hell happened with the official translation.