In ancient China, sometimes the part of speech of a word will change according to the usage in the sentence or in some special condition. It is called 詞性活用.
桂林山水甲天下, 甲 originally is used as an adjective,means the first or the best. For example, 甲級(first class). However, in here, 甲 is used a a verb which means 桂林山水is named as the best or the no.1 landscape in the world.
桂林山水天下 doesn't make sense as it just mean 'landscape of Guilin in the world'. This sentence miss a verb so it can't be a sentence and it doesn't mean anything.
Another example of 詞性活用 is 孟嘗君客我. 客 is a noun, but in here, the writer used it as a verb. That means 孟嘗君把我當作客人 (孟嘗君 looks me as a visitor.)
Of course, in nowadays, Chinese rarely use 詞性活用 in daily life.
I hope it can help you.