The dimensionless ratio between 'time of relaxation' and 'time of observation', called Deborah number (De) [1, 2], is a very meaningful concept in Rheology. The concept of viscosity is better applied to (Newtonian) fluids, for which De > 1. A gel has intermediate behaviour, having, most typically, De ~ 1. This means that the stress relaxation time has a similar order of magnitude to the experimental time scale. Gels are usually rather well modelled as viscoelastic substances, considering its rheological behaviour. The viscoelastic response of gel thin layers can be accessed experimentally (e.g. [3]).

References:

[1] M. Reiner, "The Deborah Number", Physics Today, January 1964, p. 62.

[2] C.A. Queiroz, J. Šesták, "Aspects of the non-crystalline state", Physics and Chemistry of Glasses - European Journal of Glass Science and Technology Part B, 51(3) 2010, 165-172.

[3] F. Benmouna, D. Johannsmann, "Viscoelasticity of gelatin surfaces probed by AFM noise analysis", Langmuir, 2004, 20(1), 188–193; DOI: 10.1021/la0355794

Article Aspects of the non-crystalline state

This is a textbook case in which to apply (or at least test) the Cox-Merz rule! You can take oscillatory measurements, using a low total strain (which will not damage the sample), over a range of frequency to build up the complex viscosity.

There's a nice little white paper with more explanation, attached below.

Queiroz and Miller. Many thanks for the much helpful answers