Learn English – What’s the more appropriate substitution for “give a f**k/d**n/s**t”

The word is hustler, and it can be very appropriate depending on the situation. At it's broadest, it means nothing more than "somebody who will put in the effort necessary to get things done". In the darker corners of the streets, it carries a different connotation ("one who will do anything to make a deal"), but the word lives a respectable life in the suburbs as well.

Killer may be an appropriate nickname as well -- it doesn't necessarily mean, well, a killer. It could mean nothing more than, say, a reliable closer in a sales department.

Gangster (or any street-spelling variation thereof) is probably not appropriate for the workplace -- even if your are working for, um, "legitimate businessmen" whose legitimate businesses seem to do very little actual business. They have their own preferred vocabulary, and it doesn't involve interesting any outside parties who ought to stay disinterested.

Tidings means news, or information. Glad tidings is happy news.

I'm not sure I'd use the phrase glad tidings in your example at all, to be honest. I wouldn't be giving good news to people who strive, but some kind of reward or praise.

So, for equivalents to give glad tidings I would offer:

Send good (or happy) news

For an equivalent to give glad tidings as it was used in your example I would offer:

Give praise

Or, the ever popular:

Offer large sums of cash