Air gap for inductor core
I believe that by adding 2mm on both the side gives effective air gap
of 6mm..is thst right?

Nearly correct - it produces a 4 mm air gap. You have two magnetic circuits in parallel with the centre limb being common - so each parallel magnetic circuit only has a 4 mm gap: -

Red is subject to 2x 2mm gaps and so is blue.

How does it make a difference by adding gap on legs?

That's fine - distributing the gap in several places is acceptable.

My concern is if i keep gap on both the legs like i mentioned, do i
get the AL value mentioned in the datasheet for the airgap?

Please link to the data sheet to be sure but probably, yes.