Confusing gateway problem

Hi all,
I would be obliged if someone could let me know why does specifying a default gateway in my linux mint box makes my internet connection inaccessible.The connection is via pppoe and there is no router between pppoe and my box,in short the connection is direct via dial-up (pon dsl-provider). It connects fine, only problem is that I am not able to access anything that requires internet (i.e. surfing,updating my linux box,etc; )
The same configuration on windows xp also allows the connection to be made but this time I can access anything that depends on internet connection.
I would like to know why having a default gateway on linux box restricts my internet access but the same is not the case with windows.

P.S:I am trying to build a lan in virtual box and pass the connection via pfsense (which is also a virtual machine),hence the gateway query.

P.P.S:The question has nothing to do with virtual machines or virtualization software or virtualization technology.The question relates to the problem that I am facing on my single physical machine.Hope I have made every thing clear.Do let me know if anyone wants any more clarification or information.
Thanks

Does it work if you don't specify a default gateway? PPOE is a point-to-point connection and it may be that if it works without the gateway specified it could be because the Linux system knows that network packets need to be routed to the other end of the connection, and that becomes the default gateway. FWIW, I've used PPOE with wireless modems before and I didn't need to specify a gateway.

Rubberman thank you for replying.I am sorry that I did not specified it before.Yes it works wonderfully without me specifying a gateway as the dns,ip,gateway and other stuff are provided by the isp.It still works if I specify custom ip and dns in both windows and linux.The problem arises only when I specify a gateway in linux box.The problem does not arises when a gateway is specified in windows.I believe that in windows the gateway gets replaced by the isp's gateway but this does not seem to happen in linux.
I hope I have made my case clear enough.Please do let me know if you need any more information.

Please refer to the attachments below.
I do understand the some configurations are configured by the isp.I would be grateful if someone could explain to me why does the pppoe configuration via isp differs so greatly in linux when compared to that of windows

As for why it differs between Linux and Windows, I really can't say - I just consider that if it works in Windows, it is FM (Farking Magic). If you want to change your Linux DNS entries, that is no problem - just don't change the gateway. You can change the DNS entries in /etc/resolv.conf without problems, or in the ppoe network configuration. Changing the configuration is probably better, since sometimes /etc/resolv.conf will get overwritten.