gimp is gimp but don't use the pencil tool just use the brush tool or ink tool
try FireAlpaca its free
gimp isn't really for drawing
the lines aren't really going to get smoother (pixel wise) but it looks like you are sketching you need to try just one stroke it takes practice ( you can practice with just inking your traditional work with a pen)
also you know how to use layers right? ( its very important to use them you can look up videos on youtube on how to use them) try playing around with the air brush tool for shading and highlights
this is what i made with gimp aaaaaaa while ago
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