Player controller jump not working correctly

Ok, so the jump was working perfectly, until I got a little more fancy with the functionality.

 if (GroundCheck ())
         {
             velocity.y = 0;
             gravity = Physics2D.gravity.y;
             if (Input.GetKeyDown (KeyCode.Space))
             {
                 gravity = 0;
                 velocity.y = jumpForce;
             }
         }
         else
         {
             gravity += Physics2D.gravity.y * Time.deltaTime;
             velocity.y += gravity;
         }

After adding lines 3 and 4. it hammers the player back down fast whenever the I hit space and it jumps.

I looked into the problem and it seems that the code inside if GroundCheck is running one frame after the jump.

GroundCheck:

 bool GroundCheck () {
         for (int r = 0; r < rayCountY; r ++)
         {
             Vector3 rayPos = new Vector2 (raycastOrigins.left + raycastOrigins.raySpacingY * r, raycastOrigins.bottom);
             RaycastHit2D hit = Physics2D.Raycast (rayPos, Vector3.down, skinWidth, collisionMask);
             if (hit)
             {
                 return true;
             }
         }
         return false;
     }

Basically all the ray origins are (eg left, right) are floats that determine the x position or y position of the bounds min or max of said axis, but within a skin width.

Unfortunately, I planned to get the rest of the code up on the question just in case, but we were in a rush to leave for somewhere in the day and I didn't get a chance. Hopefully the code here is enough.

Edit

I thought when I came back I would be able to fix this issue with the solution I thought I had, but it didnt fix it. New code is below:

 if (GroundCheck ())
         {
             velocity.y = gravity;
             if (Input.GetKeyDown (KeyCode.Space))
             {
                 velocity.y = jumpForce;
             }
         }
         else
         {
             velocity.y += gravity * Time.deltaTime;
         }