Continuing on from the above writeup:

It is also possible to go above the high F# on the alto saxophone, by way of altissimo fingerings. Once you get past the high F on the alto, you are considered to be in the altissimo range.

One of the fingerings for a G, above high F# is:

Octave Key
First key of the Left hand
First key of the Right hand
High F# Side Key
Bottom Side Key (used to play a side Bb)

If you use all of those keys together, you should be able to squeak out a nice High Concert Bb.