"FR's plans here must be to push MyTravel off certain routes."
Nothing wrong with that, so long as the competition is not deemed by the regulatory officals to be unfair. In fact, this preditory pricing might keep prices down for longer. Let's hope so, anyway!
With direct competition on the MJV route and indirect competition on the GRO service, FR might indeed offer lower prices than MTL for longer. These loss-leading flights will attract price-conscious travellers whilst hopefully securing a solid passenger base of loyal and frequent travellers.
In short, FR is rightly seeking to reduce MTL's market share on both routes, thereby ideally resulting in the carrier withdrawing from them both. If FR can do this without abusing its position, then good. If it cannot, i.e. by offering absurdly and unbelievably low prices because of its size which MTL cannot match (on the routes on which both carriers operate) then it must be stopped.
"all very well but how much does it cost for a taxi or bus from gro to barcelona centre??? and how long is the journey???"
GRO is about 12kms from the centre of Gerona, quickly reachable by taxi or bus.
There are 12 buses per day between the centre of Barcelona and GRO; these take about 1 hour 10 minutes.
You could, of course, take a taxi or a bus into the centre of Gerona and then a frequent train from there to Barcelona. This would, I believe, take longer.
Let's not forget that GRO does indeed serve the entire Costa Brava, not just Barcelona. As such, if you're staying somewhere in Northern Spain or further north along the coast, it might be easier, quicker and cheaper to fly into and out of GRO rather than BCN.