What is the Army Regulation that makes Government Travel Cards mandatory for all SM?

The Department of Defense (DoD) policy is that the
Government Travel Charge Card (GTCC) will be used by all DoD personnel
(military or civilian) to pay for all costs related to official government
travel. See Section 0306 for a list of exceptions. Official government travel is
defined as travel under official orders while performing duties pertaining to
official government assignments such as temporary duty (TDY) and permanent change
of station (PCS). The purpose of the GTCC is to serve as the primary payment
method for official travel expenses incurred by DoD personnel (military or
civilian) and it also allows the cardholder access to the GSA City Pair
Program. The GTCC reduces the need to issue traditional travel advances (cash/electronic
funds transfer), eliminates the need for the traveler to pay for their own expenses,
and provides financial readiness to DoD personnel (military or civilian). Refer
to the Travel and Transportation Reform Act of 1998 (TTRA), Public Law 105-264
for additional information regarding mandatory use of the travel card

SEC. 2. REQUIRING USE OF THE TRAVEL CHARGE CARD.

(a) IN GENERAL.—Under regulations issued by the Administrator
of General Services after consultation with the Secretary of the Treasury, the
Administrator shall require that Federal employees use the travel charge card
established pursuant to the United States Travel and Transportation Payment and
Expense Control System, or any Federal contractor-issued travel charge card,
for all payments of expenses of official Government travel. The Administrator
shall exempt any payment, person, type or class of payments, or type or class
of personnel from any requirement established under the preceding sentence in
any case in which—

(1) it is in the best interest of the United States to do
so;

(2) payment through a travel charge card is impractical or
imposes unreasonable burdens or costs on Federal employees or Federal agencies;
or

(3) the Secretary of Defense or the Secretary of Transportation
(with respect to the Coast Guard) requests an exemption with respect to the
members of the uniformed services.