This report describes the background calibration, inputs, and outputs of ForGATE, a forest sector greenhouse gas (GHG) accounting tool designed primarily to communicate information relevant to the evaluation of projected net GHG exchange in the context of Maine's forests, the Northeast forest sector, and alternative national or regional carbon (C) accounting guidelines. It also provides forest managers and policy makers with an easy-to-use tool for examining the relative merit (C credit revenue vs. project cost) of C offset projects and forest sector life cycle GHG accounting. GHG accounts include: 1) storage in aboveground and belowground live biomass and dead organic matter components; 2) storage in forest products in use and in landfill; 3) forest sector emissions by harvest, transport, and mills, or avoided emissions (substitution, bioenergy); as well as 4) landfill methane release and avoided emissions from methane energy capture. Different forest and forest product pools can be included in result summaries to reflect different C accounting guidelines (e.g., Climate Action Reserve, Voluntary Carbon Standard). Results can be compared for baseline and C offset project scenarios. Where possible, the marginal differences between baseline and project scenario performance indicators are calculated. All forest-level emission or storage measures are expressed in tonnes of CO2 equivalents for comparison purposes. Finally, economic indicators such as net present value and benefit-cost ratios for C offset projects can be evaluated using alternative assumptions for the value of stumpage, C credits, and offset project costs. The user enters their own inventory of stand type area by treatment regime data for baseline and offset project scenarios and can quickly adjust many GHG accounting parameters. ForGATE is available without charge from http: //www.nrs.fs.fed.us/tools/forgate/