I used to own an 04 G35 and I've done the transmission service multiple times, including pan drop and cleaning. The dipstick is secured by a bolt close to the firewall. However, other cars coming into the market no longer have the dipstick. Without transmission service, it may not last beyond 100K (which is practical in Japan). You have 2 choices - buy a dipstick kit and or have the transmission flushed (@ 12 quarts) at the dealership (@ $230). Don't let them just do a drain and refill (4 quarts).
The G uses Nissan Matic-J (Castrol Transmax J) which has been superseded by Nissan Matic-S. A quart at MSRP is $10.61 but other dealers mark up the price. At IoS,
http://www.infinitipartsusa.com. one of NICO sponsors, a quart at 25% off MSRP is @ $8.