While it's nice to have simple examples to demonstrate a tool like Gauge, it would be preferable if the examples didn't have errors in them.

The first mistake that jumps out immediately is a grammatical error. Either of these choices would be okay, but not the text that is used:

1. Vowels in the English language are "aeiou".
2. Vowels in English are "aeiou".

"English" is a noun by itself, the name of a language. It doesn't an article. But if you want to say "English language" then "English" becomes and adjective, and "language" needs the definite article "the".

The sentences are still somewhat awkward after that correction, but either adding "the" or removing "language" will fix the biggest grammatical problem.

Next is a factual error. Vowels are not defined by spelling but by pronunciation. "Gauge" does not have 3 vowels, but one!