The GS 2" 2x Barlow comes with a 1 1/4" adaptor.
A very well made 'locking' one. It fits just like a 2" EP. Slide it in and tighten the EP lock screw. It doesn't just sit there. The GS 2" is Fully Multi-Coated and has blackened inner sufaces (very important).
Here is a pic I just took showing the parts. Barlow at 2" with 1 1/4" adaptor and dust covers for each end. It is the GS 2" 2x.