I have just bought a Gasless Mig Welder and someone has said its only used for tacking. Is this right?

I agree with Bill ! but I also would like to add that maybe there also talking about weld depth , as I weld trailers all the time and use a mig to set -up /tack parts then finish with a stick welder , because I need high depth on thicker parts and even on 200 amp mig guns it just doesn't dig as deep as good ole stick does with a skilled hand

Wrong
gasless mig welders are used the same as gas /wire migs
the only difference is the polarity of the leads to the hand piece and the work ground and this is indicated by the plug in the machine for the ground lead
what the "someone " would have been referring to is the duty cycle of the unit
every welder has a duty cycle because of the heat generated in the welding process by the electronic components and transformers and if that duty cycle is exceeded the machine stops working until it cools down or damages the circuits
it is stated as a % of time in use so a unit with a 20% duty cycle means that for every 20% of use it has to cool for 80% of time
as an example ---if you were using a stick welder, the time taken to burn the stick would be 20% and the time taken to insert a new stick and clean the slag off the weld to resume the process would be the 80%
It also depends on the amps being used for the weld
on a machine with 120amps capacity and you were using 15 amps to weld sheet metal--body panels - the length of welding time will be considerable compared to using 115 amps where the weld time would be a few minutes
there are plenty of helpful hints in the operators manual and other technical publications on line