The poem starts with a simple comparison: being old the way a gate is old. A gate is weathered and worn, something people pass through without noticing, and it stays put while everything around it moves. The speaker seems to feel that kind of age, a slow, settled tiredness rather than anything dramatic.
The second line is odd and interesting. "Miles of ground lost to it" suggests that age has taken distance away, perhaps the ability to travel or cover ground, or perhaps the gate has simply watched so much land go by. The poem doesn't say which, and that ambiguity is part of its pull.
The last two lines turn to the present, which the speaker calls "hot" and sees as "a trial of little art." Judging by the wording, the current moment feels uncomfortable and clumsy, a test without much grace in it. The mood is dry, weary, and quietly wry, with a lot left unsaid in very few words.