Is is possible to trade PTS against BTC using gateway?
Short answer:
Yes
Long Answer:
Ripple will let you place any bid or ask for any currency pair on the market. For instance, I can offer to sell my 40 PTS for 1 BTC. This becomes an "ask" of 40 PTS for 1 BTC. If someone else offers to buy 40 PTS for 1 BTC, then ripple executes the trade (for no fee!) and now each party has the new currency.
The issuers of each currency have to match. If both the bid and the ask are using
XRPIO as their issuer of PTS and both are using, say Bitstamp, as their issuer of BTC, then the trade will execute.
However more complicated conversions may be available with the "convert" option. This will use markets and may go through more than one party. So, if I want to convert my PTS to BTC, I can click convert and enter to receive 1 BTC. Ripple will figure out the best path and offer to execute multiple trades automatically that will let me end up with the currency I desire. Ripple may find a path to convert from PTS => XRP => BTC through a third party even though there is no direct offer on the market for PTS/BTC. (This is a very powerful feature of ripple in my opinion.)
Because XRP is the native ripple currency, it is always trusted by everyone in ripple and can always be used as a "middle man" currency. Because of this, most bids and asks are made against XRP, especially for beginning markets, like for PTS.