The word 'Guguli' or 'Googooli' is a very slangy word for something like 'Cute' or 'Attractive' or even 'Small'.
It's used in spoken Persian. You shouldn't use it in formal contexts.
The sentence you mentioned can be translated like these:
What a cute boy/girl you were.
What a small and cute boy/girl you were.
Hope this helps. ;)