The girth is a little more tricky but in my case, my English girth (except that my dressage girth is actually shorter so it would be different if you get an english saddle with long dressage billets) is about 20" longer than the Western cinch.
I always followed the "plus 2" rule. So for an English saddle you'd be looking at an 18" seat and add 20" to the girth, roughly 48". However, you may be able to go smaller (or larger) depending on your preference.
Just remember for English saddles, more than seat size is a factor, depth of the seat flap length and flap forwardness are important considerations, too.
If you are concerned about fit issues for your gelding's back, I highly recommended talking to the staff at Trumbull Mountain Tack. They were very helpful in helping me find the right saddle for my short backed pony.
The saddle seat size is really about what you're comfortable in. I can ride in a 16in western saddle and not have a complaint but I can't ride in anything larger than a 17.5in english saddle. The flap designs and depth of seat make a difference.
As far as girths go, my horse rides in a 28in western and a 48in english. =)
English GP saddles vary enormously
different makes and models have more forward jumping knee rolls and i can ride in 16inch seat if the saddle is cut forward but more showing style GP with flatter knee rolls i wouldn't be comfortable in less than 171/2 up to 18 inches and i'm not big
also take care in width from DtoD ring at the front of the saddle and cut back withers shaping depending on the height of the wither. Theres more potential for pinching the horses back if its the wrong size.
48 inches is the magic number for a girth - seems to be that lucky average and will fit most horses (15hh roundy and skinny 16.3 will both fit a 48inch girth for me)