Guanidine HCl is soluble in water, so you can just rinse the cuvette with water to remove it. Protein residues on the surface can be removed with detergents, and there are various commercial cuvette cleaning solutions.

Here is a tip sheet on cuvette cleaning:

http://www.starnacells.com/d_tech/tech01.html

Use piranha solution, but not regularly

Aggregated proteins are quite sticky, you can use some commercial solution from Helma or Starna containing detergents, otherwise you can find some protocols for Nitric acid cleaning.

As Adam B Shapiro already said, GdHCl should not be a problem. If all else fails, protein (and lipid) can be removed from quartz surfaces with chromium sulphuric acid (5% CrO3 in conc. sulphuric acid), but that is not only very caustic, but also bad for the environment. Caro's acid (sulphuric acid and H2O2) is also dangerous, but somewhat more environmentally friendly. When using either, avoid all organic solvents, as they could form explosive peroxydes.

Engelbert Buxbaum I used routinely to use a commercial chromic acid solution for cleaning protein and lipid residues from cuvettes because it was fast and effective. I hesitated to recommend this method here, however, because of the hazards you mentioned.

Rinse it with water then HCl (diluted), then acetone and dry it by vacuum after each step

always chromic or nitric, dash ethanol in the latter if needed (care only dash)

I like to soak the cuvettes in conc. nitric acid for a couple of hours (min 2h) to over night, followed by thorough rinsing with water (5x), ethanol 70% (5x) and then finally acetone (5x), just to be sure everything is removed. This procedure can be done on a (semi-) regular basis, for thorough cleaning I recommend chromic acid every once in a while. That really does the trick!

I would advise against using acetone for cleaning, especially as last step. In my experience it leaves a whitish residue on glass surfaces.

Please look at the following below links which may help you in your analysis:

https://www.fireflysci.com/news/2015/8/4/quartz-cuvette-cleaning-find-the-best-cleaning-for-your-application

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma-Aldrich/General_Information/1/ge-protein-sample-preparation.pdf

Thanks

Use highly concentrated nitric acid for 10 min or 1 hour

or 2M nitric acid wash overnight. (Protein procepiatation would occur at high temperature )

(The above would be done when the sample is gone for thermal scanning in CD spectroscopy) followed by 7 times wash of miliq and 1 time wash with ethanol. Then, air dry the cuvvete putting inside the box. (Check, if any smear or turbidity present inside the cuvette or it can be validated by taking the spectral scan of the water in CD at required wavelength. In result, baseline of it should be straight.)

But not using the thermal scanning procedure for your samples then cleaning would be bit easy, it will require only the 7 times wash with miliq and 1 times wash with ethanol, followed by the air drying procedure by keeping it in the box itself and the further procedure after it would be same from the above paragraph.

I see that oxidative acids were recommended (chromic and nitric acids) for cuvette cleaning. I would strongly recommend you determine the method of manufacture of your cuvettes first. We used nitric acid for cleaning. It worked quite well, but the cuvette fell apart into individual slabs of quartz. As it turns out, the cuvettes were not fused into one unit but were actually joined together with an adhesive. The nitric acid did a great job of digesting the adhesive as well as the film. We had no problems with fused cuvettes. [I do not recall if they were glass or quartz, but the admonition would apply to either].