I can’t think of any phrase in the English language equivalent to ‘güle güle kullan’.
Your friend might say it to you if you bought yourself a new shirt and it literally translates as, ‘use it smilingly’.
It’s pronounced, goo-LAY goo-LAY koo-LAN. By itself, ‘güle güle’ means ‘good bye’. Exacting as the Turkish language is, you have to be the person staying behind to say, ‘güle güle!’