Hi!

I think ´günler´ is pronounced like ´günnash´.

I mean the sound "l" changes into "n".

Is there such a phonological law?

Thank you in advance. 

Definitely not. There´s already an ´n´. Perhaps it was spoken quickly and the speaker skipped or sort of skipped the ´l´.

The ´sh´ part I agree with you. This is "sometimes" done, when the word or especially the sentence ends in ´r´.

Thank you, Denizli!

Turkish people will never agree that it sounds like ´sh´/´rş´

https://www.italki.com/question/388760 

Let´s just say it´s similar but not the same.

I do not think that ´´günler´´ is pronounced like ´´günnash´´. it is interesting to hear it. 

It will be easy to understand if you can give an example.