who was very much a Russified Pole
No doubt about it. He decided to become a Russian military career officer. He was called a traitor by many Poles. [Not that this matters very much - Donald Tusk is also a traitor and murderer in Kaczyński's eyes]. I'd like to draw your attention to Positivism movement (opposing Romanticism and defeat of its ideals) in Polish literature, represented by Prus, Sienkiewicz, Orzeszkowa. After loosing two bloody uprisings (Grąbczewski's father participated in the 1863 uprising) the program of Polish positivists found a fertile ground, as they pledged: organic work, work at foundations, assimilation of Jews and other minorities, women emancipation. From their point of view Bronisław Grąbczewski could not be considered a traitor - as long as he was a Pole at heart.After passing his ensign exam in 1875, he requested transfer to the troops operating in Central Asia. As he had sworn to his father that he would not fight against his countrymen, he preferred not to remain in his home turf. At that time, Central Asia attracted people eager for adventures and contact with wild nature.
In the first three years he participated in various Russian military expeditions and missions. Those trips sparked his interest in ethnography and culture of peoples, with whom he met, therefore, he decided to ask for the release from active duty and to dedicate himself to research.
As he wrote in an article about this period of his life: "I was sitting at the rank of lieutenant - valuing more the freedom of hunting and wandering around this beautiful God's world, than all the ranks and advancement."
So much for his primary motivations.
There is no doubt that he was involved in political activities too. For example, in 1888 he masterminded - with a help of Ischak Khan, a pretender to the Afghan throne - an uprising in northern Afghanistan, directed against Emir Abdurrachman, a British puppet. The inspiration came from tsar Alexander III, who in this way, "wanted to painfully bite the British".
nevertheless lost the great game he was playing
You need to explain this to me because I know too little about the game itself. All I know that both the British and Russian empires played their sinister games in Central Asia for control of minds, trade and resources over there. I also know that both Grąbczewski and Younghusband were part of those games by definition, although sometimes it seems that they both had greater interest in explorations, discoveries and adventures, than in spy or political games.
So what was it that Grąbczewski lost, as an individual? Here is not a place to go to all the details of his expeditions. Suffice to say that his achievement in the exploration and scientific field are very impressive. I'd say he lost nothing there.
Has his career came into a halt for any political reason, such as "loosing a game"? I do not think so.
After Grąbczewski gave up further scientific travel he continued serving in Russian administration, where he attained the rank of Major General, Hetman of the Cossack armies, and governor of Astrakhan. He still travelled but only as a tourist, or to handle important business matters.
In 1896 he left Central Asia and was appointed a borderline commissioner on Amur river, and in the years 1900 -1903 he was a commissioner general of Kuantung area in southern Manchuria, leased at the time to Russia by China. He stayed In Port Arthur until 1903 - the year he resigned.
He traveled through Europe for some time. In 1907 he was appointed head of the Civil Administration of the East China Railway in Harbin. He retired in 1910 and settled in Warsaw. During World War I he was in Petersburg, then in Anapa on the Black Sea, and during the revolution he served under Gen. Anton I. Denikin on a special mission. After the fall of the Kolchak government he returned to Poland by Japan in 1920. He joined the National Institute of Meteorology, and became a member of the Polish Geographical Society and - in the term 1921/1922 - he was its board member. In 1922 he was raised to a rank of Member-Correspondent of the Society. He died on February 27, 1926 in Warsaw.