I have found that many words are only used in hiragana and because of this i can't seem to find a good way to learn them as i am studying words that use kanji due to the site i am using. Like-ごめんなさい, そうか, etc. are these type of words given a type of name?
To my knowledge, there is no term that specifically means "word usually written in kana". In modern standard Japanese, Western-origin loanwords are almost always written in katakana. Particles, adverbs and interjections tend to be written in hiragana. Learning words based only on kanji may not be a good idea because of this.