Thinking about the experience system in Traveller, and Felltower, it occurs to me that they're two completely different ways to go about it.
In Traveller, getting better is a matter of study. Adventuring doesn't do it - although it can get you Psi, so there is that. Adventuring can get you a lot, but not more skills.
In GURPS, it's a matter of getting points and spending them. In Felltower, getting those points is about looting and exploring. Mostly looting. It's loot-driven. You basically cannot get better except by adventuring.
Sometimes you do need to study:
- to learn new spells, skills, abilities, etc.
- to research information
. . . and that's about it. The first requires cash (to pay training costs - tutors, consumables, tuition, dues, etc.) and points. The latter costs cash and time.
Unless you adventure, you'll lack the cash and points needed to "study" successfully. None of them take you out of play for any time, because the point is to advance your playable paper man. No slam on systems that don't do this - this is what we intended with our XP system.