Fill about a 2.5″ gap between drywall frame and and brick

Drywall compound by itself might not be the best solution since its not really a great gap filler; it requires a backing to adhere to which is why you use either paper or fiberglass tape when putting up drywall.

As tester101 suggested, you could use crown molding or quarter round to cover it up.

I would imagine that filling it with mortar might give the most natural looking results since mortar is already used and it would blend well with the overall appearance.

Alternatively, I think you could fill this with a paintable caulking or spray foam - it really depends on the results you are going for. With caulking you are still likely to see the gap - it will just be filled with a different material now.

If your floors have their seams lined-up like the diagram (very wrong), it looks quite bad already so adding a bunch of scraps on the end won't hurt. The best solution is to literally start over & use as many of the old pieces in the manufacturer's instructions followed new & proper floors...that have at least 3 rows laid as a dry fit so you don't have any pieces shorter than 30cm.

Each row of planks has its butt seams offset from the next row & at least the same for a third row before you repeat. One main reason for this is exactly your situation here. Be careful & conscious with your measurements & cut 1 board at a time...do I leave the line, cut-out the line or split the line.

With varied lengths meeting the wall you can custom cut & even scribe each piece's end for a flawless fit or common very minor manufacturer's gap. Also, any ridges, popped nail heads, splintered subfloor faces, heavy paint or glue deposits, etc. should all be flattened, leveled, sanded, chiseled & removed prior to installing the new floor.