autonomously localise robot at "starting" point

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An alternative to gmapping is hector_slam

But @Morgan is right - you need to have a SLAM node that will take your initial position, mark it in the map and expand out from there. You can then send navigation goals anywhere wrt to this building/built map.

@Morgan , @2ROS0 worked with gmapping this much.. is there a specific function or service that i should call to give me the initial position for the robot before it even moved? wud those first laser readings really help the localisation here w/o a map built yet? or shld do an out n back to get info?

That's one of the requirements for a SLAM algorithm - to give you an initial position "even before it moved" and build from there so you don't need to give it anything like that. Yes, it will use the first laser readings to seed localization.

@2ROS0 thank you, so how do i do it in practice since i want it to be autonomous, i mean how do i call or get the initial pose in order to use it later (i.e. as the final move_base goal). or is it always (0,0)?

When you don't have a map, there is no initial pose. You can look at it like it will always start at (0, 0). Once you have the map, you can set initial pose in that map that could be the place your robot always starts in. The robot could alternatively autonomously localize in the map to get pose.

@2ROS0 thanks so much for the replies. so just to round it up for myself, when it start from scratch for gmapping i set the initial pose to (0,0) and then it builds the map around it. then when it gets the nav command to go to (0,0) it will return to point 1? thanks again :)