Martsang Kagyu is exactly still alive! The H.E. Gangri Karma Rinpoche is authorized in this lineage.
Some of people said that Martsang was absorbed by the Palyul tradition of the Nyingma. However, I have asked the Jhonkuo Rinpoche who is the highest leader of Palyul tradition of the Nyingma in Tibet, Rinpoche said that it is never to has the lineage of Martsang Kagyu in the Palyul tradition of the Nyingma. The Jhonkuo Rinpoche is over 80 years old now.
The H.H. the Sakya Trizin is one of masters for H.E. Gangri Karma Rinpoche in this life, but Gangri Karma Rinpoche is not under Sakya system! we can easy to find out the authentication from literature review of tradition Tibet Buddhism.
I don't think Rinpoche will agree that HH Sakya Trizin is just 'one of masters'. I remember he told us many times: HH Sakya Trizin is his root guru.
And if you check Rinpoche's Chinese brief biography, who are his other teachers? Khenchen Kungar Wangchuk, Luding Khen Rinpoche...
Rinpoche told us he has both Sakya and Martsan Kagyu lineage, but he mostly taught Sakya teaching in Taiwan. He also spent a lot of time doing Sakya lineage retreat etc.
It's good that the new centre in the UK mainly focus Martsan Kagyu lineage, I rejoice rinpoche's wish. However, indeed Rinpoche in this life was recognised by, taught by, and trained by Sakya system, we can't ignore that.
(But I am not very sure why HH Sakya Trizin would have involved in a Martsan Kagyu leader's recognition, must have a historical reason)
Jhonkuo Rinpoche? I think you mean Kyabje Jongkam Rinpoche tulku Tuksung (actually the same as Jamkang Rinpoche, as Rinpoche himself told me). A truly great master who was classmate of HH Penor Rinpoche. I have never heard about Martsan Kagyu lineage in Palyul lineage either.