I can only speak from my experience, which I can connect two ways.
1. I use a PSOv2 PAL GD-ROM but first I use the Sylverant boot disk, then I swap it out with the GD-ROM when prompted.
2. I really don't use this but you can use the PSOv2 autoconnect ISO. I believe it is the PAL version but I could be wrong.
I believe you can use the PSO NTSC/U GD-ROM with the boot disc as well. There is better info on Dreamcast-Live.net to help you being it has been so long since I had dealt with this.