|Sgt. Claus took signature gear for that unsat facial hair.|
Signature Gear gives equipment worth half the campaign starting wealth per point. Some pieces of equipment that might logically make sense as signature gear, such as the armor, warhorse, and weapons inherited by Dunk in G.R.R.M's The Hedge Knight, or starships like Serenity or the Millennium Falcon might work out to be extremely expensive in terms of points. Good armor could easily be worth millions of GURPS dollars, and at TL4 that's going to be hundreds of points as signature gear, which seems out of proportion to its effectiveness (you could buy it as DR with gadget limitations for much less).
There really should be a cap for Signature gear, because eventually there are going to cheaper ways to get the same item.
Signature AssetThis enhancement for Wealth on page 29 of GURPS Spaceships gives plot protection to something that you've outright purchased with starting wealth, with a cost based on the relative amount of your starting wealth you spent on it (+1% to Wealth per 5% of your starting wealth spent; with a maximum of +20% for 100%). I'm not sure I actually like this modifier, it really doesn't seem like plot protection should be worth more than a point regardless (it's essentially an Extra Option) but this does eventually get to be much cheaper than signature gear. Specifically, Very Wealthy (Signature Asset, 100%, +20%)  is cheaper than signature gear worth 20 times starting wealth  points, and you get all the other benefits of wealth like job income and imputed Status.
This suggests a cap of 36 points on Signature Gear.
PatronYou could have someone with vast assets (for example, Santa Claus) just give you the item as a Patron (Favor 1/5x) and then pay for the plot protection by taking Cosmic +50% on the Patron (or with PK's perk above). Patron (Assets worth 1000x Starting Wealth; Constantly; Cosmic: Plot Protection +50%; Equipment worth more than starting wealth +100%; Secret -50%; Favor x1/5)  points is cheaper than the equivalent signature gear (even if Santa is only willing to give you 1% of his assets, that's still 10x starting wealth or 20 points in signature gear). This has the advantage of not requiring Wealth along with it, which might not fit your concept.
This suggests a cap of 16 points on Signature Gear or 13 points if you treat plot protection as a perk.
The CapSo what does the cap mean? In the most rules-legal way it just means that you probably ought to reconsider spending more than 16 points on Signature Gear and instead consider if one of the above methods instead. If your rich uncle died and left you his tricked out vintage super-spy car, then you might be better off just taking it as a Favor from late Uncle Manfrom rather than as Signature Gear.
However, GMs might just want to consider allowing any one item as Signature Gear for 16 points, if would otherwise cost more, if it makes sense. This is especially the case when the item is expensive because of factors that don't correspond directly to game utility, like styling or rarity, it even applies to factors that do improve utility but aren't very cost-efficient such as many CF adjustments for already expensive weapons or armor.
PK's House RuleJason "PK" Levine has this rule on his MyGURPS site, which apparently is in-print in GURPS After The End 1: Wastelanders (but I don't have that book yet so I can't confirm), that divorces signature gear from the cost of the equipment completely and just makes it a perk per item (regardless of cost) to make it signature gear. Obviously this is completely incompatible with Signature Asset (above). This also, of course, gives you all the benefits of your wealth level in addition to the signature item.
The advantage of this method works both ways because with the normal rules you are incentivized to get at least 1/2 starting wealth worth of gear, which means that say a whip, fedora and revolver might not feel like an efficient use of that point.