Gideon, Ally of Zendikar's first ability, "+1: Until end of turn, Gideon, Ally of Zendikar becomes a 5/5 Human Soldier Ally creature with indestructible that’s still a planeswalker. Prevent all damage that would be dealt to him this turn."
When Gideon's first ability resolves, Gideon becomes, among other things, a "Human Soldier Ally creature" in addition to his other types (C.R. 205.1b). The text "...that's still a planeswalker" means only that Gideon's existing types are not overwritten this way, not that Gideon becomes a planeswalker if he wasn't already; see also this thread and this thread. Since Gideon is now a creature among other card types (and because Tragic Slip can target any "creature" [C.R. 115.1a]), Gideon can now be targeted by Tragic Slip; whether Gideon has other card types is irrelevant (see also C.R. 608.2i).
On the other hand, Dark Banishing can't target Nicol Bolas not because it's not a creature (which it is), but because it's black; whether it has other colors is irrelevant (see also C.R. 608.2i). Note that unlike Tragic Slip, Dark Banishing can target only a nonblack creature (C.R. 115.1a).
The part I'm confused about is the "still a planeswalker" part. Can Tragic Slip target Gideon, Ally of Zendikar since he still a counts as a planeswalker? If so then why can't cards like Dark Banishing target Nicol Bolas?
On the other hand, Dark Banishing can't target Nicol Bolas not because it's not a creature (which it is), but because it's black; whether it has other colors is irrelevant (see also C.R. 608.2i). Note that unlike Tragic Slip, Dark Banishing can target only a nonblack creature (C.R. 115.1a).