Is 128GB iPad storage enough for gaming and videos?

You would be well advised to purchase as much internal storage as your budget might reasonably allow - as internal storage (and other hardware) cannot be changed, upgraded or extended. The available internal storage is fixed for the entire life of the device. If you run out of internal storage for your documents, photos and other data, you will forever struggle and become a source of frustration. It is far better to have more storage than you need, than to need more storage than you have.

A good rule of thumb is to quantify how much data storage you will need - add a healthy margin - then double it. If you are unable to quantify your data storage needs, it becomes more difficult. Whilst I and others cannot cannot make definitive recommendations for your personal needs, objectively I would council against purchase of any iPad with any less than 256GB storage, 512GB or more providing a greater margin; 128GB should be considered an absolute minimum configuration - and this assumes you'll not be attempting to locally store large volumes of data or video on the device.

Within the Apple Support Communities we see regular tales of woe and disappointment when storage requirements are exceeded, necessitating replacement of the device; by contrast, we never see complaint of having too much available storage. It is relatively easy to fill limited internal storage space; perhaps consider higher tiers of storage as providing room for growth as your needs develop or mature - potentially extending the useful usable life of the device. 

Consider also the functional differences between WiFi Only and WiFi+Cellularmodels of iPad. Only WiFi+Cellular models include GPS capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping. Purchase of a WiFi+Cellular model does not commit you to using Cellular connections, but provides this flexibility should it be needed.  WiFi Only models of iPad have no Cellular or GPS functions at all.

Hard to say, will depend on the type of games and length of videos, but just as a comparison,

High end games like Fornite and Call of Duty can be aorund 4 to 5 GB in size to download and can use up for space once installed and being played.

Videos, if recorded in high resolution can also take up around 3 to 5 GBs or more depending on length.

I would say 128GB may be a bit too tight for that.

Using my iPad Air mostly for gaming and watching YT videos I’ve found 128GB more than sufficient.

well most apps on ios devices really rely on data not being located on the device itself

but with games and moives (unless you stream them) is all about taking up storage on your device, many users post about having issues freeing up enough space to update their device when a new version is released.

so far I have never gone with any lowest storage model and always doubled up storage with each upgrade (needless to say I do not upgrade every time they release a new model..)

I would seriously consider the 256 gb version.

If you’re planning to keep this for five or more years, marginal storage now is probably going to need ongoing management then.