I'd think so.
It seems as though the intention of Gentle Repose is to increase the time window for resurrection, in addition to general preservation of corpses. You mentioned this yourself, in the quote
effectively extends the time limit on raising the target from the dead, since days spent under the influence of this spell don't count against the time limit of spells such as raise dead. [PHB 245]
The one caveat I'd impose is the corpse needs to be Gentle Repose'd within the minute in order to still be viable. After that minute elapses (so long as you're using Revivify as your resurrection technique) you're essentially trying to preserve already-spoiled goods.
Yes you can
The main question is if an container counts its content weight towards its own weight.
There is no explicit rule that says that a container is considered to count as it's weight plus its content but I would consider it logical. Especially that the exception to this have it specifically noted see Bag of holding.
Considering this I would say that the container with content can be used for this spell as long its not being worn or carried.
The second question is how the container and content are affected by the spell.
The Catapult spell has a very clear description of how the object moves:
The object flies in a straight line up to 90 feet in a direction you choose before falling to the ground. It stops early if it hits a solid surface.
This to me means that the object isn't just flung from it's point of origin using a ballistic trajectory. In other words the magic spell interacts with the object for the duration of its movement. This would mean the spell grabs the object and moves it along the line.
This I would say means the containers content stay in the container and impact on the target always counts as one impact.
The third question then is if i use this spell to hit a vampire with a bottle of holy water or something else with Alchemist's fire. Does is do extra damage?
My answer is no. Mostly because the spell doesn't say it does. Nor would you have it do extra damage if you used the spell to launch a sword.
As a DM I might allow you to change the damage type of the spell so the Alchemist fire would change the damage type to Fire, holy water might make it Radiant, a sword could make it Slashing and Catapulting an oil lamp would make it Fire damage. But this would be a house rule for my group.
The whole answer to question 3 is very much up for ruling by your own DM. There is no rule on how this works so your DM can rule it in any way he likes.
Best Answer
Short answer
The acid test is: take your monster, and take something half the creature's size that's like it (for example, if you saw a 12ft tall giant, you would pick a 6ft tall human). If the smaller thing is significantly heavier than 60 pounds, you cannot levitate the larger thing, because the square-cube law would make that thing be heavier than 500 pounds.
Long answer
Weight stats for monsters have always been a bit of a crapshoot. Game designers, as it turns out, are usually not very good at biology.
Page 6 of the MM refers us to the PHB for rules on sizes and spaces, but the only relevant section I can find (Movement and Position) has precious little information. So we are forced to extrapolate.
The square-cube law is your friend here. Essentially, when your physical size doubles, your weight is multiplied by 8. You can use this to extrapolate weight of creatures from similar but smaller creatures whose weight is known. You might not know how heavy a Huge Giant is, but you know how heavy a Medium Human is (let's say 150 pounds for ease of math). The giant is therefore 64 times heavier - 8 times heavier for Medium > Large, and then 8 times heavier than that for Large > Huge. This puts the giant at 9600 pounds, more or less. This can vary with different types of giants (a skinny or short giant could weigh as little as 5000 pounds).
You can get more specific if you know the precise height (for example, a 12ft tall giant is only double that of a 6ft human, and if the 6ft human is 200lbs, the giant will only weigh 1600lbs). Still, this means that many Large creatures are unlikely to be affected by the spell.
Other methods
For creatures with real-life analogues of a similar size, just use the real life counterpart's weight. For example, an Aboleth is a fish, roughly the size of a great white shark. A typical great white is 2000lbs, so we can assume an aboleth is like that too. When your only concern is "more or less than 500lbs?" you don't need to be precise.
You can also try and refer to older edition materials, at least ones where the art makes the monsters look more or less the same. The D&D 3.5 SRD contains weight information on most of its monsters, and it's not terribly likely that a monster would have gotten much chubbier or skinnier over time.