enhancing a photo depends on the photo itself
first you i would suggest to adjust the brightness and contrast
(use an adjustment layer rather than changing on the photo itself)

after that you should adjust the colors,
starting with vibrancy and saturation,
then either using color adjustment, levels, or curves.
you can also use levels and or curves to adjust brightness and contrast.

after these i would clear the photo of unwanted things - using the clone stamp usually.
like zits, flashes, red eyes, scratches, any spoilers (like something totally irrelevant to the photo standing out too much), etc…
those i suggest doing in a duplicate of the layer of the original, just in case.

then duplicate the layer again and start sharpening or blurring as necessary.
if you do either of them too much, just give the layer a bit of transparency.

if you want the main thing in the photo to stand out more,
i would suggest making a brightness/contrast layer, brighten a bit, and mask it with gradient only over the thing you wanna stand out, it can be very slight and it would still make a lot of difference.

hope it was not all gibberish to you.
as i said, there is no magic, any photo needs to be taken care of specifically.

can you give me tips on enhancing ANY photo with GIMP?

ok.
it is impossible.
unless you want me to draw you a new face…

no no you don’t understand….
my question is - whats wrong with your picture that you are trying to fix?
are you trying to use it as desktop background and it is pixeled?
does it look too grainy for you?
do you try to make your face show, or to make them not showing?
do you want to fix that bright spot on your face?
are the colors too murky or are they too vivid?
there are dozens of things that can be fixed in photos,
and even more ways to fix each problem, or to reach certain effects…

and, are you satisfied with the fix i posted?
if not, why, and what do you want me to try to do?

question is, what are you trying to achieve by blurring?