G Boost doesn't activate on-call, it activates at the beginning of the attack phase, so you don't and can't activate it when you first call a monster.
Ok thanks
one more doubt, does G-Boost activate every beginning of attack phase?
Only at the start of each of your attack phases.
so for the entire attack phase, all my monsters are gboosted(base) and the critical remains +1/+2 for as many times it attacks?
Yes, it gets the boost for the whole attack phase, even if it stands multiple times.
What do you think?