Steel frame damage / repair

[SSmatty]

this is a1998 steel Rockhopper frame that I bought second hand.
At some time it has been damaged on the top tube.
It looks like it has been nicked with a cutting disc.
Is it repairable if I weld it up wit a MIG welder and then dress the weld back?
How thick is the tube likely to be?

 

[Drago]

Tube only a mm or so thick. You Can try welding, but obviously take care to avoid distortion.