Gomez is starting out as a pretty popular candidate, with 41% of voters rating him favorably to 27% with an unfavorable opinion. Beyond having good numbers with Republicans he's at 42/24 with independents, and actually seen narrowly positively even by Democrats at 33/32. Voters meanwhile are more divided on Markey, with 44% holding a positive view of him to 41% with a negative one. He's at 31/50 with independents.
For a Republican to be competitive in Massachusetts they need to win independents by a hefty margin and get a fair amount of crossover support from Democrats, and right now Gomez is doing both of those things. He's up 47/31 with independents and winning over 21% of Democratic voters. Both those numbers suggest that some folks who supported Stephen Lynch in the primary are being a little reticent about supporting Markey in the general.