Top hat laser beam Could you obtain a Gaussian profile from a square tophat laser beam?
If yes what optics should be used?
 A: With for example a neutral-density filter such as these https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1163
A: That depends on what you want to do with the light afterwards. If you require low aberration wavefronts, then you're pretty much confined to Lasper's Answer.  
Otherwise, if wavefront quality is not important (as in some illumination applications), you can use aspheric surfaces to redistribute the intensity as a function of radius. This is sometimes done to make Gaussian beams into flat tops, but the inverse is of course done to a flat top beam input to the output of such a lens. See:
D. Shafer, "Gaussian to flat-top intensity distributing lens", Optics & Laser Technology, 14, #3 1982
Google / DuckDuckGo on "aspheric intensity distribution" (separate terms, not in quotes) and this will bring you other articles.
A: You can use a spatial filtering or "beam cleaning" setup: focus the beam on a very small pinhole to eliminate all modes other than the fundamental TEM$_{00}$ (Gaussian).
You will end up throwing away most of the beam's intensity, but if that's not a concern then this is a simple low-cost solution.
