"How can I use the gap for the stack too ?"

You can't use the gap as stack, however you can force some of your variables into the gap which will free up more space for your stack.

Unfortunately Keil is unusually stupid when it comes to placing variables in the data area - it insists on grouping all the variables in a single source file into one indivisible chunk. If that chunk won't fit in a gap it is placed at the top of the pile rather than being split up. The best solution is probably to break your code up into more source files to reduce the size of each 'chunk' of variables. You could try forcing variables to specific addresses but the resulting code will be much more difficult to maintain.