Which is the best filter for the gs400? Must I also get a rubber tube for the big lens to block sun light? How much is that thing?

Just screw any UV on the cam to protect the lens along with a screw-on lens hood, that is if the GS400 doesn't already come with a hood. My MX300 came with a rubber lens hood.

I will buy any filter I find. Thank you for advise.

I had a Heliopan UV for my MX3, a slim one, and it cased flaring. I have Cokin UV's now on all my 3 cams and they work just fine. Expensive doesn't mean it's better. My Nikon UV's seem good as well, but I don't think they make them in the 37mm and 43mm size. I have some Hoya's, but not any UV's from what I can recall. They work fine.

For me the best filter for G400 will be wideangle attachment lens :-)

Good one, but which one? :-))

I don't know yet. I can't make choice witout trying and testing before.

Maybe the Raynox HD6600pro? That's the one Tom seems to prefer for his 43mm threaded MX300...if he ever puts it back together. :-))

IMO no matter which thread is attachment. Conversion rings gives ability for various thread size. I has practices with using step-down ring without any vignetting.
Therefore attachment for GS400 may be 52,49 or even (may be) 37 mm threaded.

or even (may be) 37 mm threaded.Not really. There's a good chance a 37mm--->43mm will have vignetting with the iris wide open. It's better to go larger to smaller.

For example, have a look to JVC GR-DV500:
http://www.videomax.ru/images/jvc_gr-dv500.jpg
It has 52mm screw mount. However, lens size is much less then filter thread and field of view is far narrow in tape mode. Consequently, there is no problem to use step-down ring up to 43 mm.
By the way iris don't affects to field of view.

Only tests can show is it true for GS400 or not.

I can't argue with that. :-))