Do I need a GaN adapter with USB-C PD to charge a laptop?

If you want to leave your laptop's bulky charging brick at home, yes. Reviewers on r/onebag and r/UsbCHardware consistently point to 65W+ USB-C PD as the threshold for charging most ultrabooks, MacBook Airs, and similar. Adapters in the 20–45W range will trickle-charge a laptop overnight but won't keep up with active use.