What is the property illustrated in (2+g)+3 = 2+(g+3) ?

This is the associative property of addition, not the commutative property.

The commutative property only deals with 2 numbers not 3.

The order of the numbers or their groupings do not affect the result.

(2+1)+3 = 2+(1+3)

3+3 = 2+4 (eliminate parehtneses)

6 = 6

Instead of a variable, I have substituted real numbers to illustrate how this works. You of course would eliminate the parentheses first.

The same is true of multiplication as well.

(2*4)*5 = 2*(4*5)

8*5 = 2*20 (eliminate parentheses)

40 = 40

Again, you eliminate the parentheses first and then work out the problem the rest of the way.