As you have mentioned, ventilation is good, so no need to worry about this factor of risk.
Talking about the GPU, it will be worn stronger, than on usual office work for 8-16 hours a day, so when using on 100% 24/7/365 it is unlikely it will be able to work for 5-10 years and more. But you must also consider that the GPU can have a poor design of the cooling system of the GPU itself (not a PC overall), a bad overall design, software and firmware bugs, bad production quality or production defect(s) with different severity and defect rate - from single-instance defects to massive ones. These factors can make the heating worse, cause system failure, little lifetime, short-circuiting or even might cause a fire or make you electric struck. Some factors depend on the model and the revision, some are being gradually fixed with the software/firmware updates, some vary from one single item to another. Better choose models with proven reliability reputation with a proper revision (usually the latest possible). Also, it can have a bad influence and interfere badly with the other components, for example, by generating extra electric/electronic signal noise. Also, do not forget about the fact, that the thermal paste can gradually lose its qualities and make cooling worse.
I must mention, that the graphics card is not the only component to be considered, because a PC is a complex system and its successful work depends on the state of multiple components. Every single little, even if unnecessary and unused, bad component, even the floppy drive or some decorative lights may break the PC down or cause the problems close to the ones mentioned about the GPU. For example, a bad on/off button may cause shutdown or reboot. And now more deep about the key components:
- CPU: in your use case it is likely to be used not harder than during ordinary day-to-day usage and it is likely that you absolutely do not need to overclock it. Nowadays CPUs feature all defensive mechanisms like throttling and emergency shutdown and are considered to be pretty durable. Just do not forget about the cooler and thermal paste and it is very unlikely to be the weakest point of the system.
- Motherboard: almost the same as the CPU, but there is heavy usage of PCI-e and maybe heavy usage of disks, network and peripherals, but better choose proven models.
- RAM: It is extremely unlikely to break, so this risk is not worthy of being worried about. Just use a good one.
- Disks: in the tasks that rely on disk usage (like data mining, data processing, learning a neural network with the data on the disk) HDD can become a weak point in reliability - in servers and data centres it is pretty common to change a disk in 1-3 years and very rarely "live" 5 years or more. You can use RAID 1 and backup systems to increase reliability at 24/7/365 usage (RAID 0 sacrifices reliability for performance, other RAIDs can take a lot of time to restore data. Also RAID != back up, so do not neglect with backups, if required). When using SSD, operations, that are heavy on disk-writing can drain the terabytes-written limit and make the disk useless - prefer TBW over other features. RAID 1 with SSDs can defend the system against sudden failures of one disk, but do not help with TBW rate. HDD or SSD - depends on your needs, budget and choice. Better choose models with proven reliability reputation with a proper revision (usually the latest possible).
- Power block: is heavily used by a graphics card and therefore worn more intensively - so better choose models with proven reliability reputation with a proper revision (usually the latest possible) and the power at least 1.5x more than the overall system consumption or at least 2x-2.5x more, than the main power consumers (as the GPU and the CPU). Be sure to use a good 220V AC cable, because of bad 220V AC cables are likely to cause short-circuiting, electric struck or burning (can just make smoke and self-destroy or set a real fire)!
- Ventilators: while may seem insignificant they are crucial in such use-cases and their failure is a big problem for 24/7/365 systems. Generally, install as many as you can, but also consider the size - bigger ones are quieter and more effective while the smaller ones in some cases can be installed in a bigger amount, so the failure of one single ventilator will be less painful for the system - the choice is yours.
- Exotic cooling systems: water cooling is considered to be compact and effective in high-heated overclocked systems, but water leakage can cause serious damage to PC`s components. Frozen nitrogen systems are extremely effective but likely not to be required, but are more bulky and expensive.
Professional enterprise 24/7/365 systems and components are better designed for that and have a reserve on all the components, even CPUs and BIOSes, and feature hot-replacement of components or modules, but even they do not feature 100% uptime (close, but not equal), professional Nvidia cards are faster for CUDA (especially neural networks) but I do not think it is your use case.
Assembling the system is not less important, than the components themselves. Do not forget about any single action, do not make something wrong, do not make a PC like a stupid and everything must be fine.
Make sure no software will forcibly shut down, reboot the PC or kill the process. If you are a Win10 user, you may think there is no way of entirely disabling the updates, but there are workarounds and pieces of software on the Web for that (Warning: it can violate EULA).
Peripherals can also cause problems, like the PC`s components. For example, a bad or worn mouse can register a button press when there is no press.
About key external circumstances:
- Electricity: I hope the electricity in your house is very reliable and stable because switching off electricity can make you lose the results of your work. With short-time electric problems, UPS can help you, but with more long-time issues it can give you only time to hibernate the system or to save your progress correctly.
- Network: if your task relies on the Internet or network connection, check if the wires/modem/router is ok.
Summing up: There is no solid warranty that everything will be good (literally, only death is guaranteed) and anyway you must accept the risks (they never will be equal to zero), but having a good choice of components, proper assembling and not having bad luck in buying defected components allows you use the PC that way with lower risk, then the question author initially assumed, unless you are going to do it for years and years and expect reliability for 5, 10 and more years.