びっしり is similar to ぎっしり but can also describe things written or drawn densely, like a page filled with text.
Standard kanji spelling.
Mixed kana-kanji spelling, common in everyday writing.
Rare kanji variant using 透.
From 隙間 (すきま, 'gap') + なく (adverbial form of ない, 'not'). Literally 'without gaps'.