FUNCTIONAL TESTING: 
                     it is a manditory level in blackbox 
testing,during this testing, testing team concentrating 
on "meet customer requirements" in terms of 
functonality,relability,caompatability,intersystem,configura
tion,installation,sanitation and parallel testing...etc

GUI: 
        in this we'll check the ease of use,look and feel,
spped in interface.

FUNCTIONAL TESTING :

Actually Functional testing describes two different 
aspect...It is the synonim of Black Box Testing.So in this 
context functional testing means checking the behaviour of 
the application/product without knowing the inner structure 
of the application or the product.
                              
And in different aspect Functional testing is another part 
of System Testing,which is done by the Tester....is a 
process to check or find discrepancies of behaviour with 
respect to Expected and Actual results.

Hi,

GUI Testing means Testing the look and feel, availability of
the objects, spell checks, color combination, images and 
overlapping the data like these all comes under GUI testing

Functional Testing means  there to 2 dif. ways,
1) Positive approch   2) negative approch,

based on approch we can go for functional testing.

if there is anything mistake please free to fell 
thanks