Shakespeare from around 1600 is rewarding but pretty much needs a translator into English to be understood by moderns. On the other hand Gibbon writing around the American Revolution in 1776 is modern, and is a very easy read.
The biggest issue you'll find is a variation in what moderns would consider grade level. Modern journalism is not allowed to rise about 3rd grade level reading ability because of the "need" to maximize potential readers aka profit. Bad writing drives out good writing just like bad money drives out good, or some say bad code drives out good code. (or coders?)
Gibbon is at a higher grade level of reading ability than modern average, which is going to be intimidating. I'd say its around high school level. It IS harder than the average newspaper sports page. However its not harder than any modern high school level text. Compared to a college level physics textbook, Gibbon is a pretty easy read.