Gassing

['gæsɪŋ]

His friends were gassing him up, and telling him that he was the best dancer that they have ever seen.  

G-A-S-S-I-N-G.Gassing. ['gæsɪŋ].
Gassing someone or something up means boosting someone's ego up or even making something seem cooler than what it is.In my example in the beginning, maybe the friends were making his dancing skills seem better than they were.
So if we just look at the word gas. It is fuel to keep cars going. So instead of actual gas, you are fueling up something or someone with exaggerated compliments or remarks.  
Another example is when someone says that they saw a movie that was so cool. It had so many cool actors. The quality of the movie was the best they ever saw in their who life. YOU MUST SEE IT.
So, you get excited to watch the movie, and it was not that great. It was okay. You realize that your friend was just gassing up the movie.  

Here, let us look at some example sentence:
Example 1:

She is not that pretty stop gassing her up.

Example 2:

You gassed me up so much I thought I was a great dancer. Again the word is gassing.