I’ve been wondering how gap size affects how the yoyo plays. I’ve seen differences in unresponsive yoyos as much as 2mm. I know that gap size obviously affects the difficulty of catching the yoyo on a string without hitting the sides and slowing it down. Also it seems clear that it affects responsiveness to some degree.
What else is affected by different gap sizes?
Also I did a search and couldn’t find the answers I really wanted
U talking gap size like the width of the yoyo over all or the gap of the bearing ?
The width of the gap will affect how the yoyo unwinds to the end of the string (which in turn affects perceptions of the weight of the yoyo). The gap width will also affect the bind.
Small gaps sometimes end up snagging/snaggier. I’ve found this problem on a bunch of throws, especially since I use thick string. I tend to prefer slightly bigger gaps myself.
Wider gaps can also result in snaggyness on a bind/toss, since the response isn’t acting upon a modest loop, but rather an ungodly lump of string in order to actually bite.
I like the gaps in OneDrops, except the Dietz. I think it’s too big, or my response is dying. I had a STYY Quark and that had a massive gap and it sucked to try and bind. It would snag all the time.
I now have an Aa7, it’s beefcaked with 2 A bearings and that is easier to bind than the quark.
Sometimes it’s nice to have a wider gap so you can layer string for complicated tricks but most of the time a wider gap will hurt yoh in the long run
I’ve honestly never understood what the gap size is. It obviously isn’t the width. 
Is it the width from one friction pad to the other or something else???
More or less, yes. You have the right idea. Friction pads can stick out into the gap, but assuming they’re flush, you’re picturing the right measurement. It’s just a weeeeee bit more detailed than that, but it’s too pedantic for words, when “you have the right idea” is good enough. 
I myself prefer at least a 4.3mm gap. I don’t like the gap too small due to unwanted snags or binds out of nowhere. But with a bigger gap and fat string is perfect. But dang, if seen some throws with some really wide gaps, such as the zeekio zenith with is at 5mm! That’s pretty wide for a throw if you ask me!
Yoyos can vary incredibly in gap size. For example my C3 Capless had a gap of about 5.5mm. That’s huge. In that aspect it can take as much string as you want it to in the gap, but binds can be a little slippy sometimes. Personally my sweet spot is from about 4.15 to 4.45mm
The HSPIN G&E 4 Lily came with a 5 mm wide bearing. I replaced it with a 4 mm and it greatly improved it in my opinion. Apparently others felt the same way as the Save Deth version was available with a 4 mm installed as I recall.