Volume control while converting a file

[didier]

I want to convert mp4 files whose audio track volume are rather low.
I tried increasing the value of the gain (Preferences-"all parameters" checked/audio/filters/gain). By default, the value is 1 which I have set to different values : 1.5, 3, 10 for testing purposes.
I then converted the file using file/convert procedure and choosing the "video for youtube" profile, then clicking the wrench icon to select, in the audio codec nail, the filter to use, in my case it's called "gain control filter".
This procedure does not seem to be the right one to get a significative increase of the volume of the resulting file. In fact what does the value of the gain represent ? Is it a percentage ? Are my tested values to low ?
What do you think of that ?
Thanks for your next answers !

[Rémi Denis-Courmont]

It's linear gain on the signal amplitude.

[didier]

Thanks for that answer, but clearly for the non-expert that I am, what can I expect by changing the gain default value of 1 to 2, 10 or 100 etc ?
And more important perhaps is the validation of the method I have used to increase the volume while converting. Do you agree with it ?