I think one reason I hate him now, is because I liked him so much in season 1. Been researching the show a bit and heard about the pilot film. Do I need to watch "THE GATHERING"?
Need? Not at this point, no.
Indeed you probably know enough lore by now that you might actually catch the foreshadowing/spoiler mentioned above, whereas it wouldn't have even registered if you saw the movie first. It'd be a shame to spoil that little surprise so I'd say avoid it for now, at least until you've seen the third season, it'll be safe by then.
In the grand scheme of things though, yeah, it's worth checking out in and of itself. The original aired version is a bit ropey (because: pilot) with some rather loose editing, CG that's even cruder than the first season's, some ugly make-up design and a syth-guitar sounding score that never felt like it fit the tone.
The special edition is something they did around the production of season five, when they were doing the TV movies for TNT. A lot of the editing was tightened up, some important character building scenes were re-inserted (most notably Sinclair recalling the Battle of the Line) some extraneous material was cut (like a very awkwardly staged and unconvincing tour of the alien sector, which looked like something out of Fraggle Rock) and the whole thing was re-scored to match the established soundtrack of the show.
Some of the CG was replaced too, though mostly just things like establishing shots of the station which I'm pretty sure was all stock footage from season 4. I don't think they re-rendered anything. Which is a shame in a sense because you can clearly spot what's old and what's new just by the aliasing and drop in frame-rate.
Also, just to be clear the movie is still considered canon in that the events depicted *happened*. Indeed by this point you'll already have heard some of the events mentioned on the show (Kosh being poisoned, Lyta being the original commercial telepath etc.)
Just be mindful that details like Delenn & G'Kar's makeup design, the uniforms, PPGs, links etc. are all different, to varying degrees and best not dwelt on in terms of continuity.
There was also some casting changes but those parts weren't recast so much as the characters were said to have left/been reassigned between the pilot & 'Midnight on the Firing Line'. Indeed that why the early episodes make a point of introducing Ivanova, Talia & Frankln since they were directly replacing Takashima, Lyta & Dr. Kyle respectively.
Small bit of trivia: originally Takashima was intended to have the "Control" sleeper personality, but when that leaked after the pilot, JMS decided to switch it to Talia instead of Ivanova.
Strangely while as you've already seen Lyta does come back and Kyle is mentioned in passing a few times, I don't think Takashima is ever mentioned again. The only allusion that she ever even existed is that secret coffee plant of hers that Ivanova found in hydroponics and co-opted.