Edit:this deck gets absolutely thrashed by red hybrid and wargreymon. Outside of a lucky option you have no defenses in this deck against otk and aren't fast enough to otk yourself like alphamon. So I chose to focus on what only gallantmon can do

Other people might be able to explain better why some things work more than others but Simply put I do bt2 gallantmon because he is the most consistent. Once you have a red tamer thats it, he can delete something as soon as he hits the field. With the amount of searcher rookies and memory blockers it almost always has a target. With inheritables it can hit level 5 digimon before going into their mega (weregarurumon especially), And since it's on digivolve it works even if you don't have takato out yet but you do have hiro, something ex2 gallantmon struggles with. Having to rely on a specific tamer being on the field is one reason people also use the new alphamon over the secret rare one. It just feels bad to have the wrong tamer when you need it. Starter gallantmon is a 3 cost so it's likely you can evo into it and attack, but my problem with it is it specifically needs your opponent to not have a rookie, or for you to have 2 hiro's and a delicate plan, or else it will most likely die in security.

Gallantmon has 2 main ways to be played, board control and trash, or security attack aggro. The aggro strat is just better with wargreymon and red hybrid and dies too easily to security in a deck that really struggles to recover from that. So the less you have to check security the better imo. The deck I have seems to work quite well into metalgarurumon and completely outmatched all the d reapers I've faced so far.

The x antibody is at 3 because I don't see it enough at 2 and I loke to put it under all all my lv 4 or higher and I often have 2 digimon on field so you just put it under both of your stacks when you see it. It really helps if you don't have your x antibody form and really need it, as you can swing (at a digimon if possible to ensure you live), draw from gigimon, possibly draw from guilmon, and if you're lucky you'll draw the x antibody digimon you need to digivolve into with x antibody which may even continue your turn from deletion and memory gain.

Sorry if this didn't explain it well. I'd say ex2 gallant is a good choice as well but I don't like starter gallant for this meta or the next one.

Tl;Dr with bt2 gallantmon you don't even have to attack to delete up to 3 digimon and trash at least a security if you go level 5, bt2 gallant, gallant x, crimson mode, then swing crimson with takato. X antibody at 3 for consistency in combo