What size conductor do I need to bond water and gas piping?
According to National Electrical Code, the water piping must be bonded to the service equipment enclosure, the grounded conductor at the service (service "neutral"), the grounding electrode conductor (where large enough), or directly to one or more of the grounding electrodes. The bonding jumper is to be sized using Table 250.66. For 200 ampere service, you'll probably be using 4 AWG copper or 2 AWG aluminum.

For gas piping, you can bond it to the same things as mentioned for water piping. However, the sizing of the jumper comes from Table 250.122, and is based on the size of the circuit that is likely to energize the piping.  

For example:

If you had a gas furnace that was fed by a 15 ampere circuit, the jumper could be 14 AWG copper or 12 AWG aluminum.  Since the furnace circuit is the most likely to energize the piping, you'll size the jumper based on the ungrounded conductor feeding that circuit. This is a somewhat controversial topic, as some believe the jumper should be larger.


  National Electrical Code 2014
  
  Chapter 2 Wiring and Protection
  
  Article 250 Grounding and Bonding
  
  250.104 Bonding of Piping Systems and Exposed Structural
  Steel.
  
  A) Metal Water Piping. The metal water piping system
  shall be bonded as required in (A)(1), (A)(2), or (A)(3) of
  this section. The bonding jumper(s) shall be installed in
  accordance with 250.64(A), (B), and (E). The points of
  attachment of the bonding jumper(s) shall be accessible.
  
  (1) General. Metal water piping system(s) installed in or
  attached to a building or structure shall be bonded to the
  service equipment enclosure, the grounded conductor at the
  service, the grounding electrode conductor where of sufficient
  size, or to the one or more grounding electrodes used.
  The bonding jumper(s) shall be sized in accordance with
  Table 250.66 except as permitted in 250.104(A)(2) and
  (A)(3).
  
  (B) Other Metal Piping. Where installed in or attached to
  a building or structure, a metal piping system(s), including
  gas piping, that is likely to become energized shall be
  bonded to the service equipment enclosure, the grounded
  conductor at the service, the grounding electrode conductor
  where of sufficient size, or the one or more grounding electrodes
  used. The bonding jumper(s) shall be sized in accordance
  with 250.122, using the rating of the circuit that is
  likely to energize the piping system(s). The equipment
  grounding conductor for the circuit that is likely to energize
  the piping shall be permitted to serve as the bonding means.
  The points of attachment of the bonding jumper(s) shall be
  accessible.


NOTES:
 - If this is a multifamily dwelling, the sizing might be a bit different.