Glad you are enjoying your newly acquired GP14 so much; she is a truly great design.
Transom main; agreed if you are used to a centre main the transition will perhaps take you a little time, but you will get used to it. And for cruising this has many advantages, although I concede that centre main has some benefits for racing.
There are some owners who both race and cruise, regularly in both cases, who have their boats set up so that they can quickly and easily switch sheeting arrangements depending on whether they are racing or cruising.
Spinnaker arrangements; I will leave some of our racers to advise on that, with more experience and authority that I can muster. I do use a spinnaker, occasionally, and up to my mid seventies I would even set it single-handed in fair conditions, but I am far from being an expert at it.
Enjoy getting to know the boat!
Oliver