If you are not familiar with GParted “move left and expand right” will not make much sense. GParted is like the Windows Disk Management tool. It displays the partitions on a disk as rectangles spread across the screen in a horizontal row. Check it out on YouTube.
It sounds like the Windows partitions you don’t want are likely to be to the left hand end of that row. To achieve what you want you can use GParted to delete the Windows partitions, move the Ubuntu partition(s) from the right hand end of the row of rectangles to the left and finally expand the Ubuntu root partition to take up the unused space to the right hand end of the row.
All this has to be done from a live environment. You can download a GParted iso from the GParted website and create a bootable usb device from it. Then you can make the changes described above.
If you add the output of lsblk to your question we can clarify exactly the partitions to delete and those to move and expand.
Alternatively if you are using LVM with your 100GB partition all you need to do is delete the Windows partitions, create a new partition and add it to the LVM volume. The process to do this is described here https://www.tecmint.com/extend-and-reduce-lvms-in-linux/
