I write for games professionally, albeit freelance.
I also write more technically-oriented material, and I have a bit of a background in technical writing. Mostly in clearly writing IT instructions for problem solving. You can see this in my opinion on game writing.
On top of that, one my careers is teaching. Specifically I teach mostly ESL/EFL (English as a second language/English as a foreign language), English academic/essay writing, and business and formal writing. A lot of what I use for my own writing are tools I recommend to my students, level-depending. No matter how much you want to write well and entertainingly in a second language, you need to write clearly and effectively communicate your point first. I'm constantly on the lookout for new tools that can help my students.
It occurred to me that haven't really posted about my two biggest resources for writing.
The biggest resource I use for writing is Strunk & White:
This was recommended to me by someone - I'm not sure who, now - at Steve Jackson Games. I was expected to read this and the AP Stylebook. I don't get any use of the AP Stylebook anymore. It's rare I need to check when to use a capital letter for President or which of two spellings is considered standard. I can't even tell you if I still have it - I don't keep it here, it's possible it's buried in a box somewhere.
Strunk & White - that I have right here. I periodically break it out and give it a quick read-through. It has a combination of sound explanations of how to use subordinate clauses, break out of the passive voice, format lists, and so on and great advice on writing in general. Rule #17 alone is pure writing gold:
Omit Needless Words.
It's not just pithy rules, either. The examples are clear. The rules you need are easy to find. Advice is included about breaking rules, as well.
It's $5, get a copy (or borrow one from the library.) If you do any writing at all for pay, you'll benefit from this book.
I get a lot of mileage out of Hemmingway App, too:
This is a nice editor that highlights difficult to read sentences, words that could be simplified, and adverbs. It also grades your readability. I find it superior to the built-in tools in Word and other word process I've used in the past.
I ignore the results of Hemmingway App a lot. Sometimes I can't edit out the adverbs it thinks are extraneous. Something "extraneous" is the word I need, not "extra." It really doesn't know what to do with made-up fantasy words and technical jargon. That can be useful - it helps me get rid of jargon when I don't need it.
It's a useful tool in that it visually highlights when you're being wordy and opaque. I've assigned students the task of writing an essay in it and showing me the results. The self-correcting aspect is quite nice.
If you're naturally a wordy person who learned to write by mimicking, say, THE DUNGEONMASTERS GUIDE (like me), it's a good check. It can't stop you from writing more words than you need. It can, however, warn you when you put too many in one sentence or choose them poorly.
I don't follow all of the rules in Strunk & White here. I don't use Hemmingway App here - although I sometimes use it for professional non-game writing. Sometimes I don't even proofread what I write here. This is my hobby, and getting out a post a day (or more) as I like to is enough work without needing to ensure everything is a polished gem. Still, I need resources for real work. The two I like best are those above - Strunk & White for everything, backed up by Hemmingway App when I need clarity and ease of reading.
Hopefully this helps some game writers write more clearly and well.