First you Need BIMP (=Batch Image Manipulation Plugin for GIMP) you can get it here: http://www.alessandrofrancesconi.it/projects/bimp/
after you downloaded BIMP you Need a Massive File Renamer im using this one http://ivanrf.com/en/portfolio-posts/massive-file-renamer/

Like in the Photoshop Tutorial you will need 2 Folders one with your @2x Images and one without them. like this:


Then you Open GIMP and either you put it into the One-Window-Mode:


or you let it stay in the Mode you have.

In the One-Window-Mode drag all Images into GIMP like this:


then it should look like this


after you've done that we can resize all the Images
for that we need to configure BIMP in the File Tab:

after you clicked on BIMP it should look like this

then you click on ''Hinzufügen von Bilder" Sorry im from Germany (Should be Add Images on English) after you clicked on that you either press on
all opened Images or add Folder both will work just fine, but im not sure if opened Images will work on Non One-Window-Mode if you clicked on add Folder you have to navigate to your Folder with the Images but dont go in the Folder just mark the Folder with a Left Click and Click ADD then all Images in that Folder will be in BIMP (over the "Hinzufügen von Bildern" Tab) like this:

then you click on add on top of BIMP at me ist over "Eingabedateien und Optionen" after you clicked on that you have to select "Größe Ändern" (German) in english it should be Resize after you clicked on that everything can stay like it is only Width and Height has to be changed, cuz we didnt change anything Width and Height is given in Percent we have to put both of them to 50% so the Image will be half the Size it is. Then click OK (In the Resize tab) then on the Rightside of BIMP is Save in (in German Speichern in) click on that and Navigate to the Resized Folder you Created (The Folder inside the Folder with the Images). Then click apply
after that the Images should be Resized so you can Navigate to the Resized Folder but they still have the @2x Extensions like this:

for that we need the Massive File Renamer. (You dont have to use him but its easier.)
After you opened the Massive File Renamer click on Open then Navigate to our Resized Folder (The Folder with the Resized Images) and Click on the First Image and then on the Last with Shift so you Mark all Images then click on Open then it should look like this:

then click on the Find Textbox and Type @2x you will see that on the right side (New Name) the Names Changed then Simply click on Replace All it should look like this:

then click on Replace, all Images now have no @2x at the End of their Name and our Folder Looks like this:


Thats the most efficient way i could think of so i hope it helped you and... Sorry for my Grammar i have Holidays and i just got up and im still very Sleepy but i hope it helped you our and Enjoyed it so. Have a nice day