The move 2.  P-KB4 does not threaten to take the King’s Pawn, as Black would win White’s KP by Q-R5ch.  Black can therefore develop in security with 2. ...  B-B4, and if then White prevents the Q check by Kt-KB3, there is no objection to Black protecting his King’s Pawn with P-Q3, as the King’s Bishop is already developed.  After 4.  B-B4, Black has still no need to protect his KP with Kt-QB3, but can play Kt-KB3 first, because after 5.  PxP, PxP; 6.  KtxP would be answered by 6. ...  Q-Q5 winning a piece.  Black keeps the upper hand in these early encounters because he has made a developing move with a piece, whilst White has played a pawn move which is useless for the purpose of development.

---------------------------------------
8 | #R |    |    | #Q | #K |    |    | #R |
|---------------------------------------|
7 | #P | #P | #P |    |    | #P | #P | #P |
|---------------------------------------|
6 |    |    | #Kt| #P |    | #Kt|    |    |
|---------------------------------------|
5 |    |    | #B |    | #P |    |    |    |
|---------------------------------------|
4 |    |    | ^B |    | ^P | ^P | #B |    |
|---------------------------------------|
3 |    |    | ^Kt| ^P |    | ^Kt|    |    |
|---------------------------------------|
2 | ^P | ^P | ^P |    |    |    | ^P | ^P |
|---------------------------------------|
1 | ^R |    | ^B | ^Q | ^K |    |    | ^R |
---------------------------------------
A    B    C    D    E    F    G    H

Diag. 18.

Diagram 18 shows the position which results from the following plausible moves: 

2.  P-KB4 B-B4 3.  Kt-KB3 P-Q3 4.  B-B4 Kt-KB3 5.  P-Q3 Kt-B3 6.  Kt-B3 B-KKt5

If White wishes to castle on the K side, which must have been his intention when playing 2.  P-KB4, he will have to play Kt-QR4 and KtxB.

Though this is of no disadvantage to Black, he could avoid the exchange of his KB by playing 2. ...  P-QR3 instead of B-KKt5.  If then White plays P-B5 in order to hinder the development of Black’s QB and to bring out his own, the pressure on Black’s KP is relieved permanently, and sooner or later Black will break through on the Q file, as his QP is no longer needed at Q3 for the support of the centre pawn.

A different pawn formation is the result if White enforces the exchange of Black’s centre pawn at once.  This he can do by playing P-Q4, e.g.: 

2.  P-KB4 B-B4 or 4.  P-B3 Kt-KB3 3.  Kt-KB3 P-Q3 5.  P-Q4 PxQP 4.  P-Q4 PxQP 6.  PxP B-Kt3 5.  KtxP

Here Black can get an early advantage by attacking White’s KP, taking possession of the K file after castling on the K side.

All things considered, the student should in my opinion decline the gambit, as in doing so he can get an easy and satisfactory development.  The treatment of the “King’s Gambit accepted,” which aims at holding the gambit pawn, is most difficult and leads early in the game to such complications as none but an expert can hope to master.