I cannot emphasize enough that you should
backup anything important before you resize any partitions! Seriously!
The
gparted.org website says this, "In order to add space to a partition, unallocated space must be available immediately adjacent to the partition." So, as it stands now, you cannot increase your Ubuntu partition with the free space you created.
Now, the following is a guess! Don't try it unless you are comfortable, and unless you have good backups and/or are ready to reinstall everything from scratch (which would let you create new partition sizes more to your needs if that happens). Or unless someone else gives you more knowledgeable advice one way or the other.
So, what if you added that unallocated space to your Shared Data at /dev/sda6.... and then shrink that again by 20GB? If that partition has data scattered all over, then it may not free up space "toward the Ubuntu partition" and it may simply put it back to the left where it is now. But if it does free the space up to be adjacent to Ubuntu, then you have a chance to make this work. But I am also very concerned about the dark grey shaded area at the right end of the Shared Data partition as it appears in your screen shot. I've never seen that before, and it may represent unmovable blocks that would also probably keep this trick from working.
This is all risky stuff, and I don't want you to break your system. One last thing, but you probably know this: partitions have to be unmounted to use gparted... so you will have to run gparted from Mint to attempt these changes to the Shared Data and Ubuntu partitions, if you decide to try it. Good luck! Or I hope you get a better solution!
Cheers!
