I don't think there are any catches with that offer. After the 24 months, your plan would return to the same details as it has currently. The only thing you would need to keep in mind is the date when the 24 months for that offer ends. If you get used to using the extra data, using an amount over your usual allotment after the offer ends would be considered overage.
In addition, you should note that if you decide to change your plan before the end of that offer, those extra GB might not transfer to your new plan.
Hope this helps 😀