Oh my gosh!

You're getting married soon. Your coworkers threw a surprise party for you. When you enter the party, you're really surprised. You say this.Oh my gosh... I'm  speechless.   
Oh my gosh

Say "Oh my gosh!" when you're surprised by something:

Oh my gosh! Look! She's taking her first step!

Oh my gosh! It looks like there was some kind of accident.

"Oh my gosh!" is a more polite version of "Oh my God!". Some people don't like it when others say "Oh my God!" so "Oh my gosh!" is a version that you can use around people you don't know very well.
 

I'm speechless.

"I'm speechless" means that you feel so surprised that you don't know what to say. Use this phrase when people do something really nice for you, like give you an amazing gift.

Craig, I'm... I'm speechless. Thank you so, so much!

We mostly become "speechless" because of how thankful we are. However, it's also possible to be "speechless" for other reasons, like if someone says something really dumb.

A: Is an octopus a vegetable?

B: ...I'm speechless. Are you seriously asking me that?