The concepts of gradualness and continuity are related as follows.¹ For a difference to be gradual means that given more than two elements E₁, E₂ ... E_n, E_i-1 is to E_i as E_i is to E_i+1. By extension, a certain variation whose variants differ gradually is a gradual variation, and similarly a process or a change whose stages differ gradually is gradual: stagei-1 : stagei = stagei : stagei+1. In this sense, the change from an independent pronoun via a clitic pronoun to a pronominal affix is gradual.

Gradualness is distinct from continuity. For some variation to be continuous means whichever two adjacent variants E_i and E_j are selected, there is always a variant E_k such that E_i > E_k > E_j is gradual in the sense defined. There is, for instance, continuous variation between plain and aspirated release of a voiceless stop. In the same sense, a continuous change is the diachronic counterpart to a continuous variation. Thus, some variation may be gradual without being continuous. By the above definition, all continuous variation would seem to be gradual. However, the point is that if differences between adjacent variants become infinitely small, no grades may be discerned any longer.

¹ This passage is a quote from Lehmann 2004, § 3.1. Cf. the similar distinction “between persistence and incrementality vs. continuousness and imperceptibility” postulated in Janda 2001:307f.

Reference

Janda, Richard D. 2001, “Beyond ‘pathways’ and ‘unidirectionality’: on the discontinuity of language transmission and the counterability of grammaticalization”. Language Sciences 23:265-340.

Lehmann, Christian 2004, „Theory and method in grammaticalization“. Zeitschrift für germanistische Linguistik 32(2 [2005]): 152-187. [download]