> How do you use a keyboard barely larger than a credit card?
If you can use a phone's virtual keyboard, which is even smaller and has no physical separation between the keys, you should be able to use a keyboard like that one. It feels strange at first, but you adjust quickly. (I don't have any experience with that GPD, but I have experience with an old EeePC, which is similar-sized but probably has smaller keys because it doesn't have any keys to the left and right of its trackpad.)
A phone keyboard supports swiping for words and never requires multiple buttons to be pressed. Tapping on a small target is one thing; trying to hold down a modifier without pressing adjacent keys is something very different.
I am proficient with phone keyboards, but part of that proficiency is almost never using any punctuation, numbers, or symbols that aren't on the primary screen. Once I need anything more specialized (say if I wanted to do any coding) my speed drops by a huge factor.
I can hold my phone in one hand, and type with just my thumb, and my thumb covers the full spread of the keyboard. Or I can type with two thumbs for things that are longer than a few words, and again reach the entire keyboard without shifting grip.
But also I don't type much - in fact as little as possible - on my phone, and certainly not code. I will switch to my laptop for anything more than a few words if it's possible.
If you can use a phone's virtual keyboard, which is even smaller and has no physical separation between the keys, you should be able to use a keyboard like that one. It feels strange at first, but you adjust quickly. (I don't have any experience with that GPD, but I have experience with an old EeePC, which is similar-sized but probably has smaller keys because it doesn't have any keys to the left and right of its trackpad.)