Will declined bench press help hide gynecomastia?

Not really: Breast tissue is on top of your pectoral muscle - won't resolve with whatever wt training technique you use.

No, may worsen it: Lifting weights won't help the breast tissue go away. On the contrary, bulkier pecs will push the breast tissue out further.

Probably Not.: Gynecomastia is located superficial to the pectoralis major muscle; therefore increasing the muscle size will not necessarily help diminish the appearance of the excess breast tissue. If symptomatic and/or aesthetically displeasing, surgical correction of gynecomastia can be worthwhile.