of course the cross will work. they are both squash or pumpkins.

I am sure you can find info on recessive genes on internet.

the white color is probably recessive. so here is how you might do it.

1.) make a crossing and save seeds. the first year will be all orange and then the 2nd year will be 25% white. these will be half Giant and half smaller white pumpkin. and will be larger than in 2).

2) make a crossing and save seeds. in the new year. make a crossing again with original full moon white and save seeds. You should get 75% white in the following year but there is only 25% Giant so they will be smaller.

3) to make more giant start with 1 and repeat 1 again for larger white. it will then be 75% Giant and white color. and you can keep repeating to get 87.5% Giant whites. etc etc

This can be difficult because sometimes it's not a single gene that gives the desired trait. It is possible some level of sacrifice might have to be made, breeding to get the full degree of desired traits from both parent varieties can be extremely difficult, and take many rounds of successive crossing, in some cases. It may be very simple in this situation or it may not be, hard to say.