The two simultaneous posts above by
Steve M and
HKG_Flyer1 pretty much cover everything, in particular, Steve's link.
See also Title 5, Guam Code Annotated, Chapter 73 (Customs and Quarantine Agency)
http://www.guamattorneygeneral.com/gca/5gc073.pdf
Title 4, Guam Administrative Rules and Regulations, Commerce, Chapter 2, Customs and Quarantine
http://www.naag.org/upload/04GAR001-2.pdf
Guam controls its own customs, so your routing doesn't matter. Be sure to pick up a copy of the customs form on the plane or in the baggage claim area. They are available in English and Japanese.
Your treatment will not be dependent on the grace of a "friendly customs officer."
However, whether your material is intended for personal use (gifts) or business use (operations or resale) may have relevance. I haven't read all 37 pages of the first PDF file linked to above, nor the 12 pages of the second, and am not undertaking to provide legal advice -- even though I am an active member of the Guam Bar.
In general, when filling out customs forms, I tend to list everything (collectively) I'm bringing into the jurisdiction if I intend to dispose of it, with an annotation that it is for "personal use -- gifts", plus any specified items on the form, such as alcohol.
I'll even list food items such as "packaged airline crackers," under the theory that applying a philosophy of "when in doubt, declare it" makes one less suspect by customs officials due to the attention to detail demonstrated. On the other hand, if you omit something, that makes it harder to argue it was overlooked.
Notwithstanding Guam's control of its own customs, there may be a constitutional issue as to its ability to tax items coming from the U.S., but I do not know whether it has been litigated.
I wrote most of this before reading the post of
KosraeTV, but he has more recent experience visiting Guam than I do, and it sounds right to me.
I would love to visit Kosrae some day!