The bill that authorized the 1936 (1938) Battle of Gettysburg 75th Anniversary Half Dollar was signed into law on June 16, 1936; it was approved by US President Franklin Delano Roosevelt (FDR). The net proceeds of coin sales were to "be used by [the State of Pennsylvania Battle of Gettysburg Anniversary Committee] in defraying the expenses incidental and appropriate to the commemoration of such event."
At roughly the same time, on June 24, 1936, FDR also signed into law a House Joint Resolution (Public Resolution 133) that created a Federal Battle of Gettysburg Anniversary Commission: "in commemoration of the seventy-fifth anniversary of the Battle of Gettysburg, the President of the United States is authorized to appoint a commission of five persons to cooperate with the commission appointed by the Governor of the State of Pennsylvania. The commission shall serve without compensation and shall select a chairman from among their number." The Commission was to "aid in planing for the commemoration of the seventy-fifth anniversary of the Battle of Gettysburg, and to give due and proper consideration to any plan or plans which may be submitted to them."
As the Battle Anniversary and corresponding Blue & Gray Reunion drew near in 1938 - the event was held June 29 - July 6, 1938 - Congress passed an appropriation bill to follow-up on its 1934 Commission resolution and ensure the Federal Government's involvement with the Gettysburg Anniversary events and Reunion was properly funded. The bill included a preamble and 10 additional sections.
Noteworthy sections included:
Section 2. The money herein authorized to be appropriated shall be expended under the direction of the commission appointed pursuant to the provisions of the joint resolution referred to in section 1 for the following purposes: To defray the necessary expenses of the commission in the performance of its duties, and any and all expenses incident to participation by the War Department and the Regular Army in the observance of the anniversary of the Battle of Gettysburg at Gettysburg, Pennsylvania, which participation is hereby authorized, including the expense of transportation or other movement to and from Gettysburg of individuals, troops, tentage, supplies, and equipment, and the cost of ammunition and other material expended or used incident to participation of the Army in the commemoration, including the cost of renovation or repair of material so used.With this, Federal Government participation in the Gettysburg Anniversary events was legally funded!
The US Government was also authorized to furnish transportation or reimburse travel expenses for veterans attending the Reunion:
Section 3. The commission referred to in section 2 of this Act is authorized to invite, in the name of the United States, surviving veterans, Union and Confederate, of the Civil War to reunite at Gettysburg during the commemoration, and to furnish transportation requests for travel of said veterans and attendants, in such manner as the commission may determine, or reimbursement in lieu thereof, and such per diem as may be prescribed by the commission. When rail or other commercial transportation is furnished by means of Government transportation requests, such transportation requests shall be issued over the shortest usually traveled route as determined by the commission. If travel is performed without the use of a transportation request, reimbursement shall be made in an amount equal to that which the travel of said veteran and attendant would have cost the Government if transportation in kind had been furnished at the commercial cost thereof. No veteran shall be provided for at Government expense in Gettysburg for a longer period than duration of the commemoration, except in the case of illness or other causes which prevent the return of the veteran to his home at the end of the period of commemoration. In the event of death while engaged upon the commemoration or en route thereto or therefrom the United States shall pay the cost of the preparation of the body for burial (that is, including the cost of a suitable casket) and transportation of same with escort to the home of the deceased.Congress also made sure that the veterans who attended received proper care:
Section 4. In the event that the appropriation from the State of Pennsylvania for the care of the veterans at Gettysburg is determined by the commission to be insufficient to care properly and adequately for the veterans while at Gettysburg the commission may apply to this purpose such funds from the appropriations which may be made under this authority as in its judgment are required therefor. The Secretary of War is authorized to lend to the State of Pennsylvania such property, including cots, blankets, and cooking and other equipment, under the jurisdiction of the War Department as may be available and necessary to provide for the care of veterans as above provided, and the money authorized to be appropriated herein shall be available to pay for any loss of or damage to such property and the cost of transportation and of repair or renovation thereof.The remaining Sections were primarily administrative in nature.
FDR signed the appropriation bill into law on May 16, 1938, just over a month before the anniversary was to be commemorated.
While the net proceeds from coin sales were to support the Anniversary events and Reunion, the funds were locally focused (State of Pennsylvania) and needed to be supplemented by the Federal Government for the event to be fully realized.
1936 Battle of Gettysburg 75th Anniversary Half Dollar

For other of my posts about commemorative coins and medals, including other Battle of Gettysburg stories, see:
Commems Collection.