Essentially it only moves the borders of the partitions and "repairs" the filesystem inside each affected partition.

If there is data in an area inside the partion you are manipulating gparted has to move the data to an area inside the partition that is unaffected or move it to the new parts of the partition. This can take a long time even if modern PCs easily move 100MB/s

Also, even if gparted is mature software and the devs probably have implemented a lot of security measures you should always backup your data before manipulating the partitions. Especially when you're playing around with filesystems that aren't native like NTFS or more complicated filesystems like ZFS. I know people often nag about this but trust me... Blow 2TB of your data and you really really regret not spending 10 minutes backing up the essentials.

I've been using gparted for as long as I can remember and only once or twice has it caused dats loss. Since I'm very old school (started playing with PCs when 386DX 16MHz was fairly hot and RLL disks were a thing) and nerdy I was able to use data rescue software that looked for filesystems over the whole disk and guessed where partition borders should be.

Avoid this type of anxiety by backing up all data or at least backing up the data you can't live without.

Also, if you have a spare disk, it's faster and much safer to partition the spare one and just clone each partition. Sometimes it's even faster to clone the disk this way and then clone it back.