All products in G2's universe have a default category associated with them. However, given how fungible SaaS products are across categories these days, we understand that you may want to over-ride this default category association in some cases. For instance, our Growth team has re-categorized Dropbox from its default 'Productivity' category to make it show up under 'Marketing' SaaS spend because they use it fairly heavily.
You can do the same when you're on a product's page by hovering over the category name in the Product Details sidebar and clicking the pencil icon.
Simply select a category from the dropdown and click 'Save' to apply the change.