... what if the gap is too tight, the landing pilot decided to go around for the absent of landing clearance, what do you do to deal with this situation?
Put plan B into operation.....and fast. Whenever you are controlling you have to have a way to handle things not going as you expected - in the case of arriving aircraft, always know what you will do if it goes around.
As to what to do. Well, if the weather is nice and you can tell one to turn left and the other to turn right and separate them visually it's not too much of a problem - assuming they're not one above the other! In poorer weather when you can't do visual separation you may need to co-ordinate something with approach (remember, plan ahead) in case it happens. Pass traffic information to both the aircraft so that the pilots know as much as possible about the intentions of the other - again, do it when it's begining to look like it's not going to work, not as the aircraft goes around.
But most of all, don't get into that situation. Make a decision in good time - don't let it run on until you are no longer controlling events. Easy to say, and even with years of experience you can get suckered in! But as soon as you start feeling uncomfortable be ready to use plan B.