From my knowledge of genres, Gag-a-Day fits more into Comedy, rather than Slice-of-Life.
Gag-a-day is not a genre. Gag-a-day is a popular term to describe a certain update schedule. Any comic that is based on comedic content and posts more than once a week (typically between 3-5 times per week) is a gag-a-day. There are a lot of comics that post this way, so the term was eventually coined.
Gag-a-day is comedy that is updated more than once a week. Hence "Gag" + "a-day".
Slice-of-Life is meant more for those types of comics that are literally about life; they don't necessarily have to focus on your life. They actually deal with life issues that people commonly face, from relationships, to school, etc. and often don't have a thick plot throughout, meaning you could usually pick it up anywhere and read it just fine. A webcomic I immediately think of is Live, Laugh, Love.
I think the main reason why slice-of-life has everything thrown into it is simply because people don't understand what slice-of-life is actually looking for. It's looking for the above ^^^. Whereas if your comic is more about you and your friends dicking around and focuses on jokes and such, even if your comic is about your life, that is classified as comedy.
The reason Tapastic has it set up the way they do (and this is pretty much just repeating information they've given before regarding the issue, from my own general knowledge and memory as well) is because they want to keep things simple. Plain and simple, if it doesn't offer anything to help improve the site, then it's redundant and just takes up space (although I'm putting it a little harsher than staff would). They don't want to clutter their libraries and confuse readers and creators alike by adding a MILLION different subcategories.
Think about it this way - if you'd classify your comic as a sub-category of something, why would you even need the sub-category, considering the main category does everything you need it to do by explaining what your comic is about with one word and without delving into said sub-categories?
Although this is getting dumb lol Consider if you were to sub-category fruits. Yes, there are bananas, and apples, and grapes, etc. but it's still fruit. There's no point in separating sections in the grocery store dedicated to each kind of fruit if "fruit" will literally tell you what you need to know from the get-go.
Every other issue of you finding comics categorized in areas that don't make sense is the fault of the creator not categorizing it correctly. I don't think adding 10+ other categories that you can add more than one of will help the readers find what they want to read, nor help the creators categorize their comics any better.
