it is about individual clips of course

the gate envelope does exactly what I want but I found it a tat awkward, especially because the audio-clips overlap

Volume automation has been the way to do this for as long as automation has existed. It's actually quite convenient. And believe me, I am just as lazy and impatient as you.

If I have a track with lots of short clips, the first thing I do is insert a volume envelope on the track. Then I manually insert automation nodes in front of each clip and adjust each one to balance the clip levels. If a clip is too quiet to bring it up sufficiently this way, I use Process -> Audio -> Gain to bring it up into the same ballpark as the other clips.

Yes, it's time-consuming. Everything you do for a mix is time-consuming if you're doing it well.

@ bitflipper
Process -> Audio -> Gain is the best way to go, looked at it before but it looked to complicated but now I understand it it is fine

Process > Audio > Gain is of course, a destructive process, so you should only be doing this is situations like Dave has explained, i.e. a clip is too quiet when compared to other clips.

Volume envelopes are however, non-destructive and you're free to adjust them all the way though the mixing process.

everything has hoops to jump through. I find clip gain the easiest way to deal with volume changes for clips within a track and simply ignore any affects it has on any compressors. (I can't think of anything else that would be affected.) 

If you use the volume envelope and nodes, then you have to deal with offset mode if you want to raise or lower the track volume. which, I guess, is a way to do it, I just never got used to it. 

But yes, to the OP, clip gain is the way to go if you're not anal.

Is there a way to use the "Gain" process where it does not create a stereo clip from the original Mono? I have looked over and over and see no way to toggle it. Even though it matters not as the sound will remain unchanged, it is a bit weird that the default is to stereo when ( for me) a stereo track is a rare event. I would not be overly concerned about applying gain as a destructive process... It really does no harm unless you pushed it to close or past 0. 
Because your working blind with Sonars gain process you are sometimes left in the dark as if to this has happened.    
 The problem here as I see it, is Sonar lacks the tools to analyze the clip,  report the overall and/or peak Db level, and apply the amount of gain as a Db factor that you can safely set to keep it from going over the top and distorting.. A hidden over is possible because of this. Thus the need to tool copy into wave lab to do this task right. We all have our one feature request that seems to never happen and that is mine. A real wave editor. 

>Is there a way to use the "Gain" process where it does not create a stereo clip from the original Mono?

Not sure. I have stereo and mono clips in the same track and they retain the original format after processing.

To OP: Unless you're trying to boost the volume, you can also use the fade IN and OUT nodes at the ends of each clip.

Is there a way to use the "Gain" process where it does not create a stereo clip from the original Mono?

Yes.
I don't recall the particulars. 
I'm pretty sure that is dictated by the interleave button. (or a menu choice).

must admit, it is all there, thanks for all your input

what it is all about: 
I am experimenting using Sonar as a drum-sampler. I have individual drum-samples (not full loops but individual samples for each kind of sound / hit) and I manage the samples form the arrange view. However I have not found out a clever way to deal with the "velocity".

that was also were my previous post, change playback speed of a clip , came from.