The recommended and original battery in the GL1800 is a gyz20L, which is supposedly an upgrade and the newer battery vs the ytx20hL, but nothing wrong at all with the ytx series as long as you make sure it is definately a real Yuasa. I'm sure some other brands are fine, but cheap made in China "exact replacement" is generally not a wise purchase.
You happy now ? Although I did forgot that the gyz20L was the upgrade.
Now am I going to have to go down there and help you with this or are you coming up here or are you going to do it yourself ?
Just be careful with that airbag.
I bought a Yuasa battery from my local Honda dealer for my 2005 GL1800. Nice to see them install the acid before my eyes. Certainly could not be fresher. Not sure how fresh the eBay batteries would be.