It is a habit present in the majority of MMO's to make the information readable in one glance.
In this case it means : path 3 of the dungeon, participating costs 2.5 gold
By the way this is against Arena.net politics, the gold is about paying for the dungeon run. 1-2 people that can solo/duo the run offer the last slots of the group at X gold to make profit.
In general in GW2 you can find those abbreviations too :
- AP, with for example : 5000AP min. This means 5000 Achievement Points are required to be accepted in the group
- AR, with for example 10+ AR. It stands for Agony Resistance, the statistic specific to fractals, and so 10+ means that you need at least 10 in this statistic to be accepted.
- Zerk for people requiring full Berserker gear
- 80s requires people to be at level 80 as some dungeons start being available before level 80
- Exp stands for Explorable mode, in opposition to story mode
- GC Gearcheck : people will make you link your gear to verify if it meets their requirements
And now in general in MMO's those are really frequent :
- LFG : Looking For Group (it can be LFP if you mean Party instead of Group)
- LFM : Looking For More (When a group is needing more people) (Credit to DavidYell)
- Exp : Experienced (you must know the instance/dungeon already)
(Credit to DavidYell for LFM meaning, to Amos M.Carpenter for 80s abd Exp in GW2 and to Daft for GC)