Here please allow me to list the pros and cons of buying Marriott GCs:

Pros:
1. 20% discount. We do not get additional earning from credit card since we would have earned the same points by paying with cash (credit card)
2?

Cons:
1. Commitment to a single brand Marriott. Eliminate the flexibility of stays with Hilton, IHG, Hyatt and other brands.
2. GCs are prepaid funds. The funds do not earn anything (interest). We lose the time value of money. The longer we do not use the GCs, we more we lose.
3. GC values are not protected (perishable). The physical GCs can get lost or stolen. We may forget to use the GCs. The GCs can be hacked and value drained. After the GC purchases, the credit card protection is gone.
4. GCs are not accepted at all hotels. Marriott hotels are more decentralized and it carries so many brands. Some do not follow Marriott rules and they may not accept those GCs even though they should. A lot hotels overseas do not accept the GCs.
5. GCs have limitation. At current time, they can't be used to book prepaid hotel stays. This defeats the main purpose of buying the GCs. The GC holders are not entitled for the discounted hotel rates.
6. GCs are dominated in US $ only. Even if they are accepted at foreign location, GC holders lose further more from foreign exchange. With credit card, we may get to choose to charge in USD $ or local currency.
7. GCs can't be redeemed for cash. If GC holders ever want to get the cash out of the GCs due to financial hardship, they can only be sold at the secondary markets like eBay etc. at steep costs.