I freed up some space on partition sda6 and want to merge the free space with sda8 (The partition where Ubuntu is installed)

But, when I try to shrink and move it to the right it says:

It has nothing that important in it only things I can live without.
Maybe it is the `$RECYCLE.BIN`, `Windows10Upgrade`, `.winget`, `WindowsApps` directories in the root directory.
How do I use that 100GB Space I made? Is there a safe way to do this without Windows breaking.

If I understand the partition scheme correctly, no operating system boots from that partition (sda6). Gparted often gives that warning when it can be ignored. I would go with your plan.

8bit

The best (and safest) way to shrink any partition used by windows is from within windows using the disk manager. This avoids damaging the data that may be within the space freed up. You can use the windows tools to 'optimize' the data (better known as defragging) so all data is theoretically closer to the beginning of the partition.

Once the partition has been shrunk and space is now unallocated then gparted can easily move partitions to make the newly freed space adjacent to the partition where you want to add it. Once the unallocated space is adjacent to the partition where you wish to use it then that partition size can be adjusted to add it in.

That's an important warning, all of it is true. It's up to you to decide to press OK and continue.
I'm not sure what you expect us to say.

It's always better to have a backup.

Gparted always gives that warning if one tries to move the start of a partition. The key line in the warning is: "Failure to boot is most likely to occur if you move the GNU/Linux partition containing /boot, or if you move the Windows system partition C:". The OP's scenario does not trigger either of those conditions. Having done it a hundred times, I would not hesitate to do it again.

As Caption Obvious stated, it's up to you. To be extra safe, take computersavvy's advice to use Windows to shrink it, and then Gparted to move it.

8bit

It is done successfully.