You will gain fat if you eat a calorie

If you work out quite a bit but still eat an excess- consider as an example the novice sumo wrestler, eating 5000 kcals a day and expending 4000- you will gain mostly subcutaneous fat, or fat under your skin. This isn't completely harmless, but generally the effects detrimental to you are limited to a little bit of inflammation and a strain on your joints like knees and hips, which are only "engineered" to take a certain amount of weight.

If you are not working off a lot of those calories constantly, you will start to accrue fat around your organs, which is much worse for you. Your organs will have to work much harder to do their jobs in the middle of all that fat, and you will start to edge towards lipotoxicity as your body struggles to deal with free fatty acids, which seems to play at least partially a causative role in all sorts of nasty diseases, including the insulin resistance that leads to diabetes.