Man I'm gonna have to update this when I reread lb6.1 proper. But anyways Guda in lb6 is really good but lb6.1 Guda has a very special flavor of codependancy. People often assume that Mash is the one dependant on Guda, but clearly she's balling through lb6 even away from them (well, as balling as one can be in fairy britain at least.) Guda being away from Mash however... oooh boy.
First of all, Guda gives no shit about the current plot while Mash is gone. Oberon keeps trying to egg them to go kill Morgan and such, but Guda's consistant response is essentially "idgaf WHERE is my PURPLE-HAIRED KOUHAI." Which is kinda funny actually because that is exactly how Shirou behaves in Heaven's Feel too.
Another thing is that they're way more hotheaded here. The most obvious exemples is Guda rushing past Spriggan's guards to see Mash/rushing towards the Calamity to get to her, but there are plenty of moments where Oberon and Castoria call Guda out like "dude oh my god chill tf out we can't just rush into this." Leaking a bit over lb6.2 here but it's also present in how Guda interract with Morgan. Morgan is the lostbelt king, in her own throne room, with her soldiers inches away, while Guda is in a team made of two rats and Muramasa. The smart thing would be to play nice and at least be polite in their demands. But they're not. Once business is done they just go "ok now where's Mash" while clearly pissed about it. Had it been anyone else the ruler could have just vanquished Guda on the spot for the crime of being mildly rude.
And finally, perhaps the most subtle one: Guda spends a lot of early lb6.1 making jokes about video games. "Muramasa joined the party" "everyone goes back to lvl1 here" and yes haha funny gamer momence but also to me it's very obvious that they're dissociating. They're trying to see the world as a game rather than something real to distach themself from how distressed they are. This is why when they go "oh wow this is a long sidequest chain" Castoria's reaction is concern, in a direct parallel to Guda being concerned about her when she herself dissociated earlier.
Lb6 is very much the lostbelt of Guda being mentally ill but lb6.1 is ESPECIALLY the lostbelt of Guda being on heavy Mash withdrawal and brother does it SHOW.
It is my sincere and genuine belief that Mash, a homunculus who has never stepped a foot in regular human society in her life, has a better shot at living a normal life than Guda does at this point.