Hello Dawid.
I don't know the actual weight of the G5-01, although I have just bought one, Tom at welding supplies direct kindly let me go and try one at their premises and gave me the opportunity to compare it to the similar Tecmen offering.
I can say that on first picking the G5-01 up it feels mighty heavy compared to a normal mask, but when wearing it it feels very comfortable and the weight is no issue.
I have used a speedglas helmet before and found it very unbalanced, giving near instant neck ache, but the G5-01 is very good to the point that 'in it' is a pleasant place to be. Being able to breath fresh air is a treat. No more holding your breath while grinding and welding (partly to stop steaming up) is something I'm having to learn. Having a full clear view with the adf raised is great. No more off with mask, on with goggles/ear muffs and vice versa.
The welding filter is clear and large enough in the light state to do most tasks with anyway.
It's not a helmet you want to put on for a 1 minute job but I'm impressed by how once masked up I can generally work around the workshop with the grinding visor down without it being of any great hindrance.
It was blisteringly expensive but as you can gather, I'm thinking it was worth it.
The much cheaper Tecmen V3 also looked good but the Speedglas had the edge for me. Option of a cheater lens and the optime ear defenders that I'm not sure would work with the Tecmen swung it for me.