Paying for someone else's stay
 

I will be travelling with my gf and we reserved hotel rooms under her name to be credited under her account because she is a Gold member.

I made the reservations under her CC. I wanted to take advantage of the 5X with my Chase Freedom so I was wondering if I could pay for her stay at check in so that I get my 5X and she gets her points and her stay counts towards the mega bonus?

If not, can I order some Marriott GC and get her to pay for the stays with the GC at check-in or check-out?

Yes, you can certainly do that. I am always traveling with co-workers, and I often pay for their rooms since they don't have a points-earning credit card, but they still are able to earn MR points for the stay. Win-win.

Rarely, I have had a front desk clerk give me some resistance, but that is when we are staying in separate rooms. I don't think you will have any problems at all since you are staying together.

Gift Cards
 

The easiest thing to do is buy gift cards using your CC and then use them to pay for the room. There's a sticky thread on this forum which lists discounts and deals for buying Marriott GCs.

Buy the GCs, get the points. Use them to pay for the room and your GF gets points. Everyone wins! ;)

Yes, this SHOULD work. However, it has been reported that some FDCs have stated, as do most T&Cs, the member must STAY & PAY for the room. That even a spouse paying would negate the points posting. I don't agree and I don't recall reading a resolution to the post where I read it.

Buying the GCs can be a good solution and allow you to get the benefit for future travel when the promo is over.