How safe is GParted when resizing Linux and Windows partitions?

I personally did not have a problem. I resized my disks using gparted, ntfs and fat32 and ext4. About 8-10 times until now. It should be safe, otherwise you would see problems such as "gparted broke my disk" all around the web.

The way I see it, it wouldn't be in the ubuntu distribution if it wasn't doing its job 99%. It's a well-established program with a good programmer, well-maintained and supported regularly (by closing/fixing bugs). :)

Of course, if you are afraid of losing your data, you can always buy or ask from someone to lend you a hard drive, usb or sata, so that you can backup your important data first.

As mentioned in the comments, use a livecd with gparted (e.g. ubuntu 12.04 livecd), let it do its job and don't interfere. Also, don't do a lot of steps as a batch job (don't put a lot of stuff to do in the gparted queue). Do them one-by-one and hit apply.

There's only one risky situation, if the power goes down, as per Anwar's comment:

There is nothing as 99.99% guarantee. Sudden power failure can also cause you a big metal plate, with no uses. Also note that: As per my knowledge, All partitioning tools are dangerous. They always warn their users about that

My experience with resizing partions in GParted in 100% positive. Some basic rules apply, as has been mentioned above:

• Make sure you have backup. (That's always a basic rule, not just when you're resizing partions :-)).
• Make sure your partitions are error free and unmounted cleanly. Specifically, on your dual-boot system, make sure Windows has been shut down correctly before you change anything on your partitions.

I have partitioned and re-partitioned, edited and re-sized my Hard Drive numerous times with GParted and it has never failed me. As medigeek said there is no such thing as a guarantee in these programs.

You should always take a back-up of all valuable data to be on the safe side.