Seaming garter stitch

I need to seam a pattern with garter stitch. My rows begin with a slip stitch. I do know how to mattress stitch in stockinette. My question is do I go into the first row of garter stitche to seam and not use the slip stitch to seam?

You can work mattress stitch on garter although I haven’t done it with a slip stitch edge. You could try not using the slip stitch edge (I never use it for seamed pieces) for a more invisible seam.
https://www.youtube.com/watch?v=p5GRhi-PGEg
And you can try it using the slip stitch edge and compare to see which you prefer.