“Gosh, I feel like I'm turning into a grumpy old man.”

You've been complaining to a friend about your neighbors' children, who play outside loudly in the afternoon while you're trying to relax. Now you feel guilty for complaining about that. You say:

Gosh, I feel like I'm turning into a grumpy old man.

Gosh

People say "Gosh" when they feel a strong emotion that they don't know how to express. For example, you can say "Gosh" in situations like these:

• Your boss just offered you a promotion.
• Your friend just told you that she's moving to another country.
• You just realized that it's been over 15 years since you graduated from high school.

turn into (something)

The phrase "turn into ___" means "become" but is more casual:

I'm becoming a grumpy old man.

There's another difference between "turn into" and "become". You can follow "become" with a noun phrase or an adjective phrase:

It's becoming a huge project.

They're becoming smaller and smaller.

But "turn into" can't be followed by an adjective phrase. It has to be followed by a noun phrase:

It's turning into a huge project.

a grumpy old man

"Grumpy" people complain a lot, and get annoyed or angry easily. They're hard to please.

People especially imagine old men as being "grumpy".

Being "grumpy" can be a permanent personality trait, or a temporary mood.