Simply run two cables from the G-system relay switch jacks into Formula's external switch jacks (located on the rear panel of the Formula). One of the cables must be a Y-cable ( = insert cable) from stereo TRS plug, split into two mono TR plugs. The other one can be a regular stereo TRS plug cable. Connect the Y-cable into two of the Formula's jacks, and the stereo-plug cable into the third one. No matter in which order you connect the cables to the jacks of the Formula, the relay combinations of the G-system cover all combinations of the Formula.
Note: the Formula is designed & manufactured so that the relay switching options allow multiple channels to be switched on simultaneously! (This is simply because the Lead 1 channel is the default choice, and Rhythm / Lead 2 channels are activated with differing relay combinations, which can be activated at the same time.) For example, you may scroll though the relay connection possibilities and come by the situation that both rhythm AND lead channels are switched on. Caution is advised at this point: to prevent damage to the gear, avoid running sound through the gear if more than one channel LED is on. Scrolling through, you will find all 3 channels are available with the EQ on and off.
Switching channels via the 5-pin DIN-connector on the Formula's front panel is seemingly not possible without a complexly soldered cable with resistors and such.
Thanks to Jaakko / Finland who kindly provided the information.
Please note: If you use a G-Major, you can only use 1 Y-Split cable and cover parts of the options.