I don't think this is directly related to GC fingerprints but to USVISIT program.
The law allows the entry officer to collect Biometric Identifier, and Biometric Identifier is defined as
As used in this rule, a ``biometric identifier'' is a physical
characteristic or other attribute unique to an individual that can be
collected, stored, and used to verify the claimed identity of a person
who presents himself or herself to a border inspector. To verify
identity, a similar physical characteristic or attribute is taken from
the person who presents himself or herself and it is compared against
the previously collected identifier. Examples of biometric identifiers
include, but are not limited to, the face (i.e., captured in a
photograph), fingerprints, hand geometry measurements, handwriting
samples, iris scans, retina scans, voice patterns, and other unique
characteristics.
so guys...blood, dental, rectal...every sample can come next...