"Het meisje leest gemakkelijk."
Translation:The girl reads easily.
Both mean easy. Makkelijk is makkelijker dan gemakkelijk ;). Gemakkelijk first of all, is a little more formal, but no dramatical difference, often used in written language. Makkelijk means easy: "Wat makkelijk!" - "How easy" or imagine the package of an easy-to-prepare-meal saying "Lekker makkelijk, klaar in 2 minuten!", or so. Gemakkelijk can also be used in the idiomatic expression "Maak het u gemakkelijk" - make yourself (feel) comfortable.
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That's a close call, if I understand "gemakkelijk" well enough. I understand the Dutch sentence to mean that she is able to read so well that reading is easy for her. Ik kan gemakkelijker Engelse tekst lezen dan ik de Nederlandse tekst kan lezen. "Comfortably," on the other hand, typically means that she is physically content while she reads: in a comfortable chair, for example.
However, we can speak of "reading comfortably" to mean essentially the same as "reading easily": she is so good at reading that she is at ease--comfortable--when she does it. (This seems more often used in places like, "He is comfortable speaking in front of large crowds.")
I think "comfortably," in that sense, would work. But I also think it's a close call.