I have like a thousand pictures that I would like to flip them horizontally in GIMP, do you know how I can achieve that?
I guess I will have to use some kind of a batch process, but I'm really not familiar with it.
You can use ImageMagick or GraphicsMagick:
magick mogrify -flop *.png gm mogrify -flop *.png
GraphicsMagick seems to be about 70% faster for this operation on a directory full of out-of-the-camera JPEGs.
Use -flip to flip top-to-bottom instead of left-to-right.
Note that the images will be recompressed by "mogrify". If you are working with JPEG images, you can use the "jpegtran" application (which is part of the libjpeg distribution) to rotate or flip images losslessly. If you are working with PNG or GIF, this recompression is unavoidable.