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Some conceptual issues concerning general invariant theories, with specialemphasis on general relativity, are analyzed. The common assertion thatobservables must be required to be gauge invariant is examined in the light ofthe role played by a system of observers. Some features of the reduction of thegauge group are discussed, including the fact that in the process of a partialgauge fixing the reduction at the level of the gauge group and the reduction atthe level of the variational principle do not commute. Distinctions between themathematical and the physical concept of gauge symmetry are discussed andillustrated with examples. The limit from general relativity to specialrelativity is considered as an example of a gauge group reduction that isallowed in some specific physical circumstances. Whether and when thePoincar\'e group must be considered as a residual gauge group will come out asa result of our analysis, that applies, in particular, to asymptotically flatspaces.Comment: 17 page

Observables, gauge invariance and the role of the observers in the limit from general relativity to special relativity