Rearrange partitions in gparted to extend a partition

You are right it is the grub2 core.img you can fix this by moving grub2 core.img to the end of the unallocated section and the you will be able to resize sda1. You should be able to edit it the same way you are with sda1 and then drag the block to the end then resize sda1 also check out this answer as it is similar to what you want.