Assuming that you mean federal tax credits for energy efficiency - the American Recovery and Reinvestment Act (ARRA) increased the energy tax credit for homeowners who make energy efficient improvements to their existing homes. The new law increases the credit rate to 30 percent of the cost of all qualifying improvements and raises the maximum credit limit to $1,500 for improvements placed in service in 2009 and 2010.
The credit applies to improvements such as adding insulation, energy efficient exterior windows and energy-efficient heating and air conditioning systems.
Here are technical requirements - http://www.energystar.gov/index.cfm?c=tax_credits.tx_index - scroll down for details.
Gas fireplaces are not in this list. Please verify if any other items in the list may be used in your situation.
Let me know if you need any help.