Hmm I looked at the gumroad, really dislike the way the author releases their stuff. Can someone explain which is the best platform to support him on? I like using gumroad but the way NT releases this series separately leaves a bad taste in my mouth. Supporting him on Tier 2 Patreon will allow all access for NT's previous work? Does he at least after each month give a promo code to claim his content? (Like kidmo)
The Gumroad releases are more like back issues for none subscribers, or as a way for those that joined late, to be able to purchases his previous works. If you make a pledge on Patreon, he sends you a private message every month to access that month's rewards.
And no, pledging for 1 month doesn't give you access to all of his previous works, only tier 1 stuff is freely accessible. Tier 2+ you'll need to purchase previous from Gumroad. This is common practice for many artists (kidmo does this too since you mentioned him), you can't expect unlimited access to previous stuff when you've only paid for 1 month's worth, or people would just pledge for 1 month, cancel, wait for 6 months or thereabouts, and repledge for 1 month and get access to everything from 6 months ago.
As for why Gumroad, again this is fairly common practice, since Patreon doesn't allow locking out post by month, only by tier level. Many artists use Gumroad to get around this problem. Another artist I follow, Ginhaha, (/tag/artist:ginhaha) does things the same way. Fantia doesn't have this issue, since the artist can lock out posts by tier grade as well as by month, and also supports paying to buy access to previous months. But Fantia is Japanese based, so all the stuff on there are all censored, but his tier 3 plan is cheaper, 700 yen, which is about $7 USD.