As is the Boy Scouts of America.
To understand things, you need to understand the early history of boy scouting in the USA. After Scouting became popular in the UK, where it was founded, a number of competing scouting programs cropped up in the US in the 1900's. Not only was there the Boy Scouts of America, there was also the American Boy Scouts (which was backed by William Randolph Hearst), the US Boy Scouts, the Boy Scout Navy, the Rhode Island Boy Scouts, and other such organizations. There was a struggle for legitimacy that took place among the rival organizations that famously included an attempt to divert (same say "kidnap") the founder of Scouting, Sir Robert Baden-Powell, from going to a BSA public diner held in BP's honor to a rival event of the ABS when he arrived in the US for a visit. In a matter of years, and after well publicized scandals involving ABS leaders pocketing funds donated to their organization, the feuding ended when Congress granted the BSA a charter (effectively making them incorporated by the US government) and granting exclusive use of the term "Boy Scout." The GSUSA and the Red Cross are other examples of organizations with Congressional charters. The WOSM will only recognize one organization per country as a member organization, and they picked the BSA as the "boy" organization in the US.
I'm not educated as to the history of the GSUSA, but they developed on a separate path of their own. I'm not sure when, but at some point the BSA and GSUSA signed a memorandum of understanding that neither organization would offer a program for members of the opposite sex under the age of 14. It has operated under that agreement since then. I don't know about the GSUSA, but in 1969 the BSA allowed girls to join their older-scout (age 14 to 21) "Explorer" program.
The move that the BSA just announced has been hinted at for several months now. Not surprisingly, the GSUSA, has been openly hostile to the idea from the get go. The national leader of the GSUSA sent the head of the BSA
a blistering letter in August.
As noted, the US is one of the few countries where the boy and girl scouting programs don't roll up in to one parent organization. What this move means for the GSUSA isn't fully clear, but isn't not hard to see it as an effort to siphon off members to add numbers to the BSA balance sheet. While the BSA has seen a significant drop in membership, they are still in a much better position that their GSUSA counterparts. The GSUSA also has a major problem with pension liabilities to retired workers.