So tools that can really help you add that nice finishing touch to your sig would be the Blur tool/ Sharpen tool/ Dodge tool/ and Burn tool!~ Those tools are very useful for that finishing touch! Also try messing around with the "Curve" settings and the "Channel Mixer". What is also very useful is finding a really nice texture and putting it on top of your sig and mess with the settings, I tend to like to change it to "Soft Light" and lower the opacity if needed. I hope that makes a little sense hahaha If you have any questions, feel free to ask XD
As for the text, don't worry about it ahahah everyone has troubles with text when they begin. I had LOTS of trouble with it too when I first started. It just honestly takes time to find that right one. What I would suggest is to download a wide variety of fonts you like from dafont.com, so you have a variety to choose from. Also try to add some effects to your text as well. There are some tutorials for GIMP on deviantart about text. C: