How can expand ext4 while it is inside an extended partition

You don't have any space for expansion, extended partition or not. GParted would be able to resize a logical partition just fine¹, but it cannot merge the contents of two filesystems into one.

If you want to merge both partitions into one, you will need to:

1. Back up everything from /home to another disk (either as files, or as a whole partition image);
2. Remove /home from the /etc/fstab file;
3. Boot into a LiveCD (preferably one which has GParted on it);
4. Delete the sda6 (home) partition;
5. Grow the sda8 (OS) partition to the empty space on the right;
6. Restore all your files from backup.

If you want to adjust the sizes, but keep the partitions separate, you will need to:

1. Back up everything from /home to another disk (either as files, or as a whole partition image);
2. Boot into a LiveCD which has GParted;
3. Shrink the sda6 partition from the start (this will be a slow process as it needs to shift all data);
4. Grow the sda8 partition to fill the new empty space.

Although these tasks could sort-of be done from a running system (ext4 can be grown while mounted), it would still involve closing all your programs, probably even stopping the GUI and carefully typing numbers into fdisk in console mode. Doing the resize from a LiveCD will be quite a bit easier.

---

¹ But if your firmware allows, you really should consider switching to UEFI boot mode and GPT disk partitioning which does not need "extended" or "logical" partitions at all.