Jack Lucchetti and I started adding MIDAS support to gretl in June 2016. A reasonably mature version of this support can be found in gretl 2017a (released in April 2017), though improvements are ongoing as of gretl 2017c.
The file midas_gretl.pdf contains provisional documentation for MIDAS in gretl. This contains some illustrative scripts, but a longer sample script is available: gdp_midas.inp replicates a set of ADL-MIDAS examples from Eric Ghysels' MIDAS Matlab Toolbox. The examples in question are described in section 4.1 of the User's Guide for the Toolbox; the Matlab code can be found in the Toolbox file appADLMIDAS1.m. Native Matlab output is in matlab_output.txt and gretl output in gretl_replic.txt.
The dataset used in the replication exercise (quarterly US GDP plus monthly payroll employment) is available in gretl format: gdp_midas.gdt. (This file is now included in the gretl package.)
A supplementary discussion of some issues connected with MIDAS forecasting can be found in midas-supp.pdf.
In the course of developing gretl's MIDAS apparatus I have found it helpful to be able to run the MIDAS Matlab Toolbox on GNU Octave. This requires some minor modifications to the Toolbox files. The document midas_octave.pdf gives details, and the archive MIDASv2.0_octave.zip contains kit to make the required modifications.
I've also compared results with the midasr package for R (documentation here). Some relevant files:
Rsim.inp and Rsim.txt: Gretl script (with embedded R) to replicate a simulated MIDAS example from section 3.2 of the midasr doc. The simulation includes two high-frequency series with different frequencies and different parameterizations. Plus output.
Allin Cottrell, Wake Forest University, September 2017.