In the most general sense, a gap is a break or an opening. It can be a physical opening, like a gap in a fence, or it can be an empty space or interval, like a gap in a siren’s wailing. A gap can also be an interruption in continuity: a gap in one’s memory. Just like gap, the synonym chasm refers to an opening or break, but on a much larger (and considerably more ominous) scale: in the literal sense, a chasm is a yawning fissure or deep cleft in the earth's surface—you might also call it a gorge. But chasm is more commonly used to talk about situations in which there is a very wide distance, or more specifically, a marked difference between two things—almost to the point of being impossible to bridge or to bring together. You might encounter it used to describe an irreconcilable breach in relations marked by a divergence of opinions or beliefs between persons or groups (a chasm between the haves and have-nots).
So, how hard is it, really, to file those tax returns? The adjectives grueling and taxing both describe activities or experiences that are tiring and difficult. We tend to reserve the adjective grueling for activities or experiences that are extremely difficult to complete or endure (the grueling Boston Marathon; a grueling schedule), to the point of being severe or punishing. Something that is taxing wears on a person gradually, by the demands it makes on the body, mind, or emotions (the most emotionally taxing role of the actor’s career). The word taxing is more amenable to qualification—something could be a bit taxing or extremely taxing, whereas grueling is pretty much an all-or-nothing adjective.
The verbs agree and concur both mean to have the same opinion as another. However, agree often presupposes an interpersonal process by which this harmony is reached. The context might involve discussion and negotiation (they agreed on a theme for the party), or consent or compliance (agree to a pay cut) between the parties. To concur, on the other hand, is to show accord in matters of opinion, as of minds that have independently run along the same channels (to concur in a judgment about a painting). It would be unusual to say, “I concur with you,” not only because concur is a more formal word choice than agree, but also because concurring is more impersonal: one doesn’t usually concur with a person but with the substance of an opinion or judgment.