GP means LPDDR3 support which capped out at 16gb on intel, which was because their 10nm lines with LPDDR4 were delayed years. It was always possible to use regular DDR but for laptops the low power variant is important.
Yes they do on the 16”, after years of complaints from users who didn’t understand the constraint, and which they added the maximum allowable battery size to (~100wH), and it still only lasts 3-4h in moderate use.