Monster Gate & Last Will

[Dragonseeker]

Could someone please help me understand how this card combo should be resolved?
I'm trying to wrap my head around this whole Magical Scientist deck, First Turn Kill, etc.

Scenario:

I summon one monster to the field. [say, Mother Grizzly]
I activate Last Will.
I activate Monster Gate, and tribute Mother Grizzly for the effect.

What happens now? Can I resolve Monster Gate first, to see what comes out
[hopefully a Catapult Turtle] and THEN choose a monster for Last Will [Magical
Scientist]? Or is it reverse, where I have to choose one of the two monsters above
and hope the other one comes out.

Or, is all the above illegal in any way?

Thanks in advance!

 

[densetsu_x]

Since you are in the middle of resolving "Monster Gate", you do that first.

Then, you search for your monster with an ATK of 1500 or less.

- A