I miss my GT.....
right, that out of the way, Congratulations!
I personally found the GT more than adequate when my daughter was born. The boot is plenty big enough (several family holidays, never any need for more space) and I never had a problem with 3 doors either.
Maybe it's just me, but the extra width of the doors helped with access. With a 4/5 door car you have to reach over and around the infant carrier to click the buckle, with a 3 door you sit in the car, facing it to click it, which I always found easier.
Admittedly if you suffer with back problems, you might have an issue.
The key is to make sure you get the right pushchair/pram (best advice I can give here from experience, irrelevant to what car you have/choose) Not too small that you don't have room, and not too big that it won't fit in anywhere.
When junior is older (9 months+) make sure you get the smallest seat you can, preferably an iso fix one, as we could only find 1 seat (a britax) that we deemed safe enough. At this stage, we found getting her in and out even easier. No seatbelt to worry about with iso fix.
To be honest I guess you have to try it. Yes, a sportwagon would be more practical, but would it be as much fun?
Much as I loved my GT (actually didn't buy it until AFTER my daughter was born) if it turned out that it really wasn't practical it would have gone.
When I bought the car, we took the infant carrier, pram etc with us to the dealer just to make sure it fit, and to see how easy it was getting her in and out etc. (yes, I had to convince Mrs Zulu that it was safe/practical etc, and she was happy.
Mind you, I took my daughter for a week in cornwall in my GTV.. now that was a tight squeeze! At least she is older now, and doesn't need so much stuff ..... just shoes (takes after her Mum!