Sadia Ibrahim

Typical crystal lattices (anisotropic structures) are studied using the XRD method. Gels are isotropic structures, like polymers. In the picture you can get a halo like polymers if you make a thick gel by drying. See the halo of the polymer in the article

Article Physico-Chemical Properties of Nanoparticles Functionalized ...

You can test a wet sample by xrd. However, you have to control the sample under test per quantity/volume/and depth for consistency of the test method. In addition, you should perform multiple tests without changing your sample, at least twice, to see the impact of evaporation on your data.

I am assuming that the hydrogel is a solid and that you do not want the water to evaporate from it (if it is a liquid, XRD will be of little use). I have found that you can keep a solvent such as water from evaporating from a sample during measurement by immersing the material in mineral oil in a capillary or under a glass cover slip (depending on the geometry of your measurement). The oil and glass will contribute some background in the measurement but not much.

It is really subject to your analytical purposes. I did a lot of investigations on the batches of suspension samples by xrpd for clinical trials.