Do you have a schematic drawing of tile layout?

How about flipping the tile?

Use the grey glossy to line the niche that's in the matte white wall and use the matte white tile to line the niche that's in the grey glossy part of the wall.

This may sound odd, but you could use the tiles "as is" or cut them down to mimic the scale of the wall tiles.

Example, the grey glossy wall tiles are 4x8. So line the niche with white mattes that are cut down to 2x4, 25% the size of the 4x8s.

Same for the other. The white matte wall tiles are 2x8, so line that niche with grey glossy tiles cut down to 1x4, again, 25% the size of the wall tile.

Obviously it depends on a lot. Can you cut the tiles. Cut edges. Grout lines, etc.

Just an idea. Good luck!