How to measure wavelength of gamma-rays

The direct method is by crystal diffraction.

Here is an article by Kessler et al (1988):

High accuracy, absolute wavelength determination of capture gamma-ray energies for E≤5 MeV and the direct determination of binding energies in light nuclei

This is a long comment, for clarity of terms used:

without knowing the exact energy and without measuring energy,

The term "gamma rays" used as a term describing a classical electromagnetic wave of specific frequency and wavelength is fine.

The term "gamma" is attributed to a photon, a quantum mechanical particle of the standard model, which is a point particle, thus has no wavelength , and has mass zero. Its energy is given by $hν$ where h is the Planck constant and $ν$ is the frequency of the corresponding classical electromagnetic wave when one has a large number of photons. When the energy of a gamma photon is known, then the wavelength of the classical wave it will contribute to if many gammas of that frequency are accumulated ( see this to see how single photons even one at time build a classical interference pattern) can be calculated.

i.e. sort of direct measurement.

Knowing the energy of a single gamma photon is a direct measurement, and the continuity of mathematics between classical electrodynamics, and quantum electrodynamics ensures that is so, so it is a direct measurement of the wavelength too

Of course the experiment linked by Peter is also a direct measurement, from the classical side.