Notepad++ is a fabulous software program that had no complete alternatives on Linux. I used it for scripting, text manipulation, note taking, dumping and editing thoughts. Scintilla-based equivalents Geany, SciTE exist, but do not come close.
Really? No alternatives on Linux? Have you tried Emacs? I think Emacs with Org mode blows Notepad++ out of the water in all the uses you just mentioned.
If you want a similar editor on Linux, then I suggest Kate. If Vim and Emacs didn’t exist, I’d be using Kate.