Here is a Commonweal obituary.
The obit contains a couple of minor inaccuracies.
1. "Under his father's tutelage, one of Geach's earliest philosophical influences was the metaphysician J.M.E. McTaggart, who infamously argues in his 1908 book The Unreality of Time for, well, the unreality of time." This title is not a book but an article that appeared in the journal Mind (17.68: 457–474), in 1908.
McTaggart presents a full dress version of the famous argument in his 1927 magnum opus, The Nature of Existence, in Chapter XXXIII, located in volume II.
McTaggart's argument for the unreality of time is one of the great arguments in the history of metaphysics, an argument as important and influential as the Eleatic Zeno's arguments against motion, St. Anselm's ontological argument for the existence of God and F. H. Bradley's argument against relations in his 1893 Appearance and Reality, Book I, Chapter III. All four arguments have the interesting property of being rejected as unsound by almost all philosophers, philosophers who nonetheless differ wildly among themselves as to where the arguments go wrong. Careful study of these arguments is an excellent introduction to the problems of metaphysics. In particular, the analytic philosophy of time in the 20th century would not be unfairly described as a very long and very detailed series of footnotes to McTaggart's great argument.
2. "Along with Aquinas and McTaggart (whose system he presents in his 1982 book Truth, Love, and Immortality), Geach's main philosophical heroes were Aristotle, Ludwig Wittgenstein, and Gottlob Frege." My copy of Truth, Love and Immortality shows the University of California Press (Berkeley and Los Angeles) as the publisher and the publication year as 1979. The frontispiece features an unsourced quotation from McTaggart:
The longer I live, the more I am convinced of the reality of three things -- truth, love and immortality.