Antigravity Lifting Gas

The simplest way to work with this is to assume that the gas has a negative mass. Such a gas would produce greater lift if more of it is used, so if your gas cells are of a given size, there is an advantage in compressing it and using very high pressures. It also provides lift by the buoyancy effect, since it is displacing an equal volume of air. For greatest buoyancy, you want as large a gasbag as practical.

The following gives an idea of what sort of gas pressure are likely to be feasible at various Tech Levels:

TL5-:

You're looking at gasbags made of animal skin or sailcloth or something similar, perhaps coated with tar or resin to make it gastight. Either way, this material is not going to hold much of a pressure difference before ripping. Maybe 1.1 or 1.2 atmospheres if you're lucky.

TL6:

The era of Real World(tm) zeppelins. These had gastight bags with 1 atmosphere of hydrogen. However, I suspect at that TL they could have coped with an overpressure up to about 1.5 atmospheres, by using metal banding around the gasbags to keep the stressed bag areas small and so relatively stronger.

TL7:

You're looking at fully enclosed metal gas containers here. Titanium or an aluminium alloy would be the choice for lightness and strength. Strength will depend on the thickness of the metal. You might reasonably get up to the 2-3 atmospheres mark with a fairly thin sheet (think aluminium drink can or a bit thicker), but this will be quite fragile and easy to rupture. Go to centimetre or so thickness of metal and you're probably talking 10s of atmospheres, maybe up to 100. Of course the problem then is the weight of the metal itself - though with enough overpressure of antigravity gas, you'll more than compensate for it.

Late TL7:

Advanced structural cloths make an appearance (kevlar, etc). These can probably get up to the holding strength of thin metal - a few atmospheres.

TL8:

Structural cloths will get stronger and lighter. You might have a cloth gasbag capable of holding 10-20 atmospheres.

TL9-10:

Further improvements in materials mean lighter gasbags capable of holding ever more pressure... perhaps 100 or more atmospheres.

TL11:

Force fields become the containment method of choice. For the weight of a power source and force field generator, you can contain almost arbitrary levels of pressure. If you use the GURPS Vehicles "crush depth" rules to determine how much DR you need to withstand a given pressure difference (A calculation I do not fully agree with... see Vacuum Lifting Cells) then it comes out to roughly DR1 per atmosphere. So DR1000 of force field could hold about 1000 atmospheres of overpressure.