The first step is knowing your device's maximum supported charging power, usually listed on the phone's spec sheet. Buying a 100W charger for a phone that only supports 20W will not cause harm, but it is an unnecessary expense; what matters is matching the power level to actual need.
Deciding on port count depends on how many devices you charge at once. A single port is usually enough for one person, but a household or a traveler charging multiple devices at the same time will save real time with a two- or three-port GaN adapter.
On certification, look for CE marking along with clearly stated overcurrent and overheat protection. If a product page or box does not mention this information clearly, it is fair to question whether the product ever passed proper safety testing.
Physical size also matters; GaN models deliver the same wattage in a much smaller housing, which takes up less room in a travel bag or on a desk. Finally, brand reliability and warranty length are decisive for long-term satisfaction, and brands like Charjett that offer real technical support have an edge here.