How do I merge the 8 GB of "free space" on Macintosh HD under Logical volume group into Macintosh HD?
Resizing a Logical Volume in a larger Logical Volume Group with some free space is done with the undocumented coreStorage-verb resizeVolume:

diskutil cs resizeVolume lvUUID size


with lvUUID: UUID of the Logical Volume (the UUIDs are retrievable by entering diskutil cs list) and size (size in b/k/m/g/t | 0 | R)

0 and R are magical sizes and try to maximize the size of the Logical Volume (depends on the system version - doesn't work in all macOS systems).

In your case I recommend:

diskutil cs resizeVolume FD192884-33A4-43CB-86A6-9CE4E58B3B92 120128m


If you get an error disk to small (or similar) choose slightly smaller sizes like 120064m or 120000m until you are successful. The final size of the expanded Logical Volume is slightly smaller than the underlying Physical Volume: some space on the PV is required to store CoreStorage meta data.

Example:

Fusion Drive 3.12 TB:

combined PV size:  3120722 MB
LV size:           3106192 MB
∆ (PV-LV) size:      14530 MB (= 14.5 GB or 0.47%!)