Describe "The Scholar Gypsy" as a pastoral elegy.

An elegy is a poem reflecting on an important, often sombre theme, usually someone's death. In relation to poetry a pastoral is a piece evoking the blissful joys of a heavily romanticized rural life, a bucolic idyll populated by nymphs, shepherds, and cavorting wood sprites. A pastoral elegy, then, combines elements of both kinds of poem to create a work that movingly laments someone recently deceased, presented in the guise of a shepherd. One of the most popular examples of the sub-genre is Milton's "Lycidas," in which a late fellow student of his, one Edmund King, is portrayed as a shepherd inhabiting a dreamily lush Arcadian landscape.
Matthew Arnold, in writing "The Scholar Gypsy," utilizes the traditional elements of pastoral elegy, but at the same time develops them in a slightly different direction. The rural life which the wandering scholar is now thought to lead is presented favorably by comparison with the world he's left behind, a world in which the passage of clock time, the endless moments "exhaust the energy of strongest souls/And numb the elastic powers."
But having renounced his former life, the scholar gypsy has achieved immortality, no longer subject to the deadening hand of time and all it brings:

Free from the sick fatigue, the languid doubt,
Which much to have tried, in much been baffled, brings.
O life unlike to ours!

No less than in other examples of pastoral elegy, the countryside is presented by Arnold as a timeless haven from the cares of the mortal world, a place where free spirits such as the scholar can truly live, move and have their being. The simplicity of rural life with all its natural, diurnal rhythms is contrasted with the "sick hurry" of the modern urban world.

The scholar gypsy is ageless because he cannot succumb to the "strange disease of modern life." He's left all that behind. Arnold breaks with the tradition of pastoral elegy by treating the scholar gypsy as if he were alive. Although he's been rumored to wander the countryside for the better part of 200 years, he's still portrayed in the poem as being very much alive in spirit, his rare appearances down the years glimpsed by shepherds and other country folk. It is the truthfulness of their testimony which gives the poem's speaker the confidence to assert the continued existence of this "truant boy."
https://www.poetryfoundation.org/poems/43606/the-scholar-gipsy