In TaxSlayer Pro, enter Form W-2GU as foreign employer compensation.
Guam has its own tax system based on the same tax laws and tax rates that apply in the United States.
With one exception, all individuals with income from Guam will file only one return, either to Guam or to the United States. The exception is a nonresident alien filing Form 1040-NR with the US will also file a tax return with Guam reporting only their income from sources in Guam.
A citizen of Guam who is not otherwise a U.S. citizen or a U.S. resident alien during the tax year files an income tax return with Guam. Similarly, a bona fide resident of Guam files a tax return with Guam, irrespective of whether they are a citizen, resident alien, or nonresident alien of the United States.
Otherwise, a taxpayer who is not a bona fide resident of Guam files a tax return with the United States, and they won't be liable for filing a return with or paying tax to Guam.
See IRS Publication 570 for more information about income from Guam, return mailing addresses, and special rules.
Form 5074
The US and Guam use Form 5074, Allocation of Individual Income Tax to Guam or the Commonwealth of the Northern Mariana Islands (CNMI) to divide a taxpayer's income taxes. A taxpayer filing a U.S. income tax return needs to complete Form 5074 if their AGI is $50,000 or more for the tax year and they received gross income of $5,000 or more from sources within Guam.
Form 5074 is not included in TaxSlayer Pro but can be completed and printed to either paper or PDF for attachment at the IRS website here.
Additional Information:
IRS: Publication 570, Tax Guide for Individuals With Income From U.S. Territories