why will my 2005 gmc sierra truck start sometimes and sometimes not?

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I have a 2005 gmc sierra truck and sometimes when I go to start it the truck will not start. It is not the battery because everything comes on and the windows will roll up and down. If it doesn't start and I let it sit for a little while it will eventually turn over and start.It can go for days and start fine but then there are days that it will have bouts of starting or not starting. Also my fuel gauge doesn't work.