So as you can see the unallocated space is right next to my /home partition, my question is, can I just merge them by resizing sd3 (/home partition) with GParted right away? Or do I have to use a Live CD? I got a warning message saying that something might go wrong.
Using a LiveCD is safer. I agree with C.S.Cameron that you should have a backup, because working with filesystems and partitions can destroy data.
Changing the size of a partition is only part of what you need to do in order to make this space usable.
Once you expand the partition to use the unallocated space, you need to resize the filesystem (the ext4 aspect of this) to also include the new space. Fortunately, ext4 can be resized online.
After you've expanded the partition, close gparted and open a terminal.
sudo resize2fs /dev/sda3
This should tell your ext4 partition that it's bigger now, and allow it to use the new space.
For good measure, I'd reboot.
