Is there “Else” condition in GD? I only found “And”, “Or”, “Not” in Advanced conditions.
No, there is not a direct way, but (of course) you can emulate it with a variable:
[code]No conditions | Do = 1 Variable(else)
—Object_A collide with Object_B | Actions if A collides with B
Do = 0 Variable(else)
—Variable(else) = 1 | Actions if A doesn’t collide with B[/code]
There is another, easier way: just check for opposite condition. So if you e.g. have condition Variable A = 35, check for in next “else” condition NOT Block (Variable A = 35). No need to waste a variable.
Check a variable is less expensive than check the most of GD conditions. In my example you have to check one variable instead a collision detection (collision detections are very expensive, but not a big difference with few events).
But imagine multiple conditions, how do you make it inverting the conditions? I mean, for example you have the conditions:
A and B and not C:
your elses could be A and B and C, A and not B and not C, not A and B and not C (and 2 more useless cases), or some particular logics with OR and AND (variables in each event) 
The variable system should be less expensive, easy, general, and the performance will be not affected by the number or type of conditions 
I have yet to see GD’s performance to be affected by number of events.